the following description is merely exemplary in nature and is not intended to limit the present disclosure , application , or uses . it should be understood that throughout the drawings , corresponding reference numerals indicate like or corresponding parts and features . in general , the present disclosure relates to a coupling and hypoid disconnect system for a driveline of a motor vehicle . a power take - off unit may be equipped with a synchronizer to disconnect the power source from a portion of the driveline and to reconnect through synchronization of the driveline . a dog or roller - type clutch may be provided to disconnect a portion of the driveline from one or more of the vehicle wheels . additionally , a friction coupling may be positioned in series within the driveline to provide speed synchronization between front and rear driveline components when a power reconnection is desired . the hypoid gearing of the vehicle driveline may be separated from the driving source of power to reduce churning losses and other mechanical inefficiencies . with particular reference to fig1 - 3 of the drawings , a drive train 10 of a four - wheel drive vehicle is shown . drive train 10 includes a front driveline 12 and a rear driveline 14 both drivable from a source of power , such as an engine 16 through a transmission 18 which may be of either the manual or automatic type . in the particular embodiment shown , drive train 10 is a four - wheel system incorporating a power transmission device 20 for transmitting drive torque from engine 16 and transmission 18 to front driveline 12 and rear driveline 14 . power transmission device 20 is shown as a power take - off unit . front driveline 12 is shown to include a pair of front wheels 24 individually driven by a first front axle shaft 26 and a second front axle shaft 28 , as well as a differential assembly 32 . power take - off unit 20 includes a reduction speed gearset 30 , a synchronizer clutch 34 , an output gearset 35 and a right - angled drive assembly 36 . rear driveline 14 includes a propeller shaft 38 connected at a first end to right - angled drive assembly 36 and at an opposite end to one side of a friction coupling 39 . the opposite side of friction coupling 39 is connected to a rear axle assembly 40 . rear driveline 14 also includes a pair of rear wheels 42 individually driven by a first rear axle shaft 44 and a second rear axle shaft 46 . rear axle assembly 40 also includes a hypoid ring and pinion gearset 48 driving a differential assembly 50 . a disconnect 52 selectively drivingly disconnects second rear axle shaft 46 from ring and pinion gearset 48 and differential assembly 50 . reduction speed gearset 30 of power take - off unit 20 includes a drive gear 56 fixed for rotation with an output shaft of transmission 18 . a driven gear 58 is in constant meshed engagement with drive gear 56 and is also fixed for rotation with a carrier 60 of differential assembly 32 . differential assembly 32 includes a first side gear 62 fixed for rotation with first front axle shaft 26 and a second side gear 64 fixed for rotation with second front axle shaft 28 . each of first and second side gears 62 , 64 are in meshed engagement with pinion gears 66 , 68 which are rotatably supported by carrier 60 . power take - off unit 20 also includes an input shaft 76 supported for rotation within a housing . input shaft 76 is fixed for rotation with carrier 60 of center differential assembly 32 . a drive gear 78 is supported for rotation on second front axle shaft 28 . a driven gear 80 is in meshed engagement with drive gear 78 and fixed for rotation with a ring gear 82 of right - angled drive assembly 36 . driven gear 80 and ring gear 82 are fixed for rotation with a countershaft 84 . synchronizer clutch 34 selectively drivingly interconnects input shaft 76 and drive gear 78 . synchronizer clutch 34 includes a hub 86 fixed for rotation with input shaft 76 . an axially moveable sleeve 88 is in splined engagement with hub 86 . a second hub 90 is fixed for rotation with drive gear 78 and includes an external spline 92 . synchronizer clutch 34 also includes a blocker ring 94 positioned between hub 86 and second hub 90 . blocker ring 94 functions to assure that the rotational speed of input shaft 76 is substantially the same as drive gear 78 prior to allowing a driving connection between hub 86 and second hub 90 via sleeve 88 . it should be appreciated that an alternate synchronizer ( not shown ) may not require a blocker ring to function properly . a synchronizer clutch actuation mechanism 96 includes a shift fork 98 slidingly positioned with a groove 100 formed in sleeve 88 . an actuator 102 is operable to move fork 98 and sleeve 88 from a first position where sleeve 88 is disengaged from spline 92 and a second position where sleeve 88 concurrently drivingly engages hub 86 and second hub 90 . right - angled drive assembly 36 includes ring gear 82 and a pinion gear 108 in meshed engagement with ring gear 82 . pinion gear 108 may be integrally formed with a pinion shaft 110 . pinion shaft 110 is fixed for rotation with propeller shaft 38 via a flange 112 . synchronizer clutch 34 may be placed in an activated mode where torque is transferred between input shaft 76 and drive gear 78 . synchronizer clutch 34 is also operable in a deactivated mode where no torque is transferred to rear driveline 14 . power from engine 16 is not transferred to right - angled drive assembly 36 when synchronizer clutch 34 is in the deactivated mode . friction coupling 39 is depicted as a friction clutch fixed to a rear axle assembly 113 . rear axle assembly 113 includes differential assembly 50 , rear axle shaft 44 , rear axle shaft 46 and disconnect 52 . differential assembly 50 includes a carrier housing 114 fixed for rotation with a ring gear 115 of ring and pinion gearset 48 . differential assembly 50 also includes first and second side gears 116 , 117 fixed for rotation with first and second rear axle shafts 44 , 46 , respectively . a pair of pinion gears 118 are positioned within carrier housing 114 and placed in constant meshed engagement with side gears 116 , 117 . friction coupling 39 includes a drum 120 fixed for rotation with propeller shaft 38 . a hub 122 is fixed for rotation with a pinion shaft 124 . a pinion gear 126 of pinion gearset 48 may be integrally formed with pinion shaft 124 . outer clutch plates 128 are splined for rotation with drum 120 . a plurality of inner clutch plates 130 are splined for rotation with hub 122 and interleaved with outer clutch plates 128 . an actuator 134 is operable to apply a clutch actuation force to clutch plates 128 , 130 and transfer torque through friction coupling 39 . in one example , an axially moveable piston may be in receipt of pressurized fluid to provide the actuation force . alternatively , an electric motor may cooperate with a force multiplication mechanism . in yet another embodiment described below in greater detail , the friction clutch may be actuated based on wheel slip or a difference in rotational speed across the friction clutch . disconnect 52 is depicted in fig1 and 3 as a dog clutch . disconnect 52 includes a first hub 140 fixed for rotation with a shaft 142 drivingly engaged with side gear 117 of differential assembly 50 . an external spline 146 is formed on first hub 140 . an axially translatable sleeve 148 is in splined engagement with first hub 140 . a second hub 150 is fixed for rotation with rear axle shaft 46 . a spline 152 is formed on an outer periphery of second hub 150 . a dog clutch actuation system 156 includes a fork 158 slidably positioned within a groove 160 formed in sleeve 148 . an actuator 162 is operable to translate fork 158 and sleeve 148 between a first position where sleeve 148 is engaged only with first hub 140 and a second position where sleeve 148 simultaneously engages splines 146 and 152 to drivingly interconnect shaft 142 with rear axle shaft 46 . fig3 a depicts an alternate rear driveline 14 a and rear axle assembly 40 a . rear axle assembly 40 a is substantially similar to rear axle assembly 40 , previously described . accordingly , like elements will retain their previously introduced reference numerals . rear axle assembly 40 a includes another disconnect identified as disconnect 52 a . the elements of disconnect 52 a are identified in similar fashion to the components of disconnect 52 except that the suffix “ a ” has been added . disconnect 52 a may selectively drivingly connect and disconnect rear axle shaft 44 with an axle shaft portion 142 a that is fixed for rotation with side gear 116 . during operation , ring and pinion gearset 48 and differential assembly 50 may be entirely disconnected from rear axle shaft 44 and rear axle shaft 46 . accordingly , even the internal components of differential assembly 50 will not be rotated due to input from rear wheels 42 . to return to the all wheel drive mode of operation , actuator 162 a is controlled at substantially the same time as actuator 162 to reconnect shaft 142 a and rear axle shaft 44 in the same manner as shaft 142 is coupled to rear axle shaft 46 . fig4 and 5 depict an alternate drive train 10 ′. drive train 10 ′ is substantially similar to drive train 10 . as such , like elements will be identified with the previously introduced reference numerals including a prime suffix . drive train 10 ′ includes a power take - off unit 20 ′ that differs from power take - off unit 20 by being a single axis power transmission device that does not include countershaft 84 , previously described . on the contrary , power take - off unit 20 ′ includes a concentric shaft 166 having ring gear 82 ′ fixed thereto . ring gear 82 ′ is in meshed engagement with pinion gear 108 ′ to drive propeller shaft 38 ′. fig4 and 5 also show that disconnect 52 may be alternatively formed as a roller clutch identified as reference numeral 52 ′. the driveline depicted in fig1 and 4 may include either a dog clutch , a roller clutch or one of a number of other power transmission devices operable to selectively transfer torque and cease the transfer of torque between rotary shafts . in the example depicted , roller clutch 52 ′ includes an inner member 170 fixed for rotation with rear axle shaft 46 ′ and an outer member 172 fixed for rotation with shaft 142 ′. inner member 170 includes a surface 174 having a plurality of curved recesses . each recess is in receipt of a roller 176 . a split ring 178 is positioned between rollers 176 and outer member 172 . split ring 178 also includes a plurality of curved recesses facing the recesses of inner member 170 and in receipt of rollers 176 . a control arm 180 cooperates with split ring 178 to restrict or permit relative rotation between inner member 170 and split ring 178 . when relative rotation is permitted , rollers 176 are forced radially outwardly to radially outwardly expand split ring 178 into engagement with outer member 172 to transfer torque across roller clutch 52 ′. when relative rotation between inner member 170 and split ring 178 is restricted , rollers 176 are not displaced , the rollers are not wedged between split ring 178 and inner member 170 and torque is not transferred across disconnect 52 ′. an actuator 182 may move control arm 180 to operate disconnect 52 ′. during vehicle operation , it may be advantageous to reduce the churning losses associated with driving ring and pinion gearset 48 as well as right - angled drive assembly 36 . with reference to fig1 , a controller 190 is in communication with a variety of vehicle sensors 192 providing data indicative of parameters such as vehicle speed , four - wheel drive mode , wheel slip , vehicle acceleration and the like . one sensor 192 may be positioned at a location proximate ring and pinion gearset 48 to provide a signal indicating the rotational speed of a ring and pinion gearset component . at the appropriate time , controller 190 may output a signal to control actuator 96 and place synchronizer clutch 34 in the deactuated mode where torque is not transferred from engine 16 to rear driveline 14 . controller 190 may also signal actuator 162 , associated with disconnect 52 , to place fork 158 in a position to cease torque transfer across disconnect 52 such that the energy associated with one of rotating rear wheels 42 will not be transferred to ring and pinion gearset 48 or differential assembly 50 . accordingly , the hypoid gearsets 36 , 48 will not be driven by differential assembly 32 . furthermore , because side gear 116 is not restricted from rotation , input torque provided by rear axle shaft 44 will only cause the internal gears within differential assembly 50 to rotate . ring and pinion gearset 48 is not driven . it should be appreciated that friction coupling 39 may be operated in either of an open mode or a torque transferring mode when synchronizer clutch 34 and disconnect 52 do not transfer torque because rear driveline 14 is not rotating at this time . when controller 190 determines that a four wheel drive mode of operation is to commence , controller 190 signals actuator 102 to slide sleeve 88 toward hub 90 . during this operation , speed synchronization between input shaft 76 and drive gear 78 occurs . once the speeds are matched , sleeve 88 drivingly interconnects hub 86 and second hub 90 . at this time , right - angled drive assembly 36 is also driven by engine 16 . once the front driveline components and the right - angled drive components are up to speed , controller 190 provides a signal to actuator 134 to begin speed synchronization of ring and pinion gearset 48 as well as differential assembly 50 . this sequence of operations will cause the speed of shaft 142 to match the speed of rear axle shaft 46 . at this time , controller 190 provides a signal to actuator 162 to place disconnect 52 in a torque transferring mode by axially translating sleeve 148 . at the end of this sequence , drive train 10 is operable in an all wheel drive mode . it should be appreciated that the procedure previously described may be performed while the vehicle is moving . it is contemplated that friction coupling 39 may be alternatively configured as a passive device having an actuation system operable in response to a speed differential between propeller shaft 38 and pinion shaft 124 . in particular , fig4 depicts friction coupling 39 ′ including a pump 202 driven by propeller shaft 38 when a speed differential exists between propeller shaft 38 ′ and pinion shaft 124 . pressurized fluid from pump 202 is provided to a piston 204 for applying a compressive force to inner clutch plates 130 ′ and outer clutch plates 128 ′. in this arrangement , control of synchronizer clutch 34 ′ also provides control of friction coupling 39 ′ because rotation of propeller shaft 38 ′ relative to pinion shaft 124 ′ will cause pressurized fluid to cause torque transfer across friction coupling 39 ′ thereby quickly achieving speed synchronization of the front driveline and rear driveline components . furthermore , the inclusion of friction coupling 39 or 39 ′ allows synchronizer clutch 34 or 34 ′ to be relatively minimally sized because only some of the components of power transmission device 20 and propeller shaft 38 are speed synchronized through actuation of synchronizer clutch 34 . the relatively large rotating masses within rear axle assembly 40 are accelerated through actuation of friction coupling 39 . while a number of vehicle drivelines have been previously described , it should be appreciated that the particular configurations discussed are merely exemplary . as such , it is contemplated that other combinations of the components shown in the figures may be arranged with one another to construct a drive train not explicitly shown but within the scope of the present disclosure .