Regional Jet 700 ER/LR (CRJ700)
Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Provided Assistance; Flight Crew Diverted; Flight Crew Took Evasive Action; General Flight Cancelled / Delayed; General Maintenance Action
Aircraft
Descending into ZZZ on the arrival north transition. I believe we were in ALTS (???) mode at 6;000 ft; approximately 210 knots; autopilot ON when the stab trim clacker began to alert for what I'd estimate was 5-6 seconds. In my experience; this was an  unusual time and aircraft configuration for the trim clacker to sound; much less for the duration it sounded. I think both the Captain and I had the inclination to do the Stab Trim Runaway items and proceeded to do so. We ran the full Stab Trim Runaway from Immediate Action Items to QRH. During this process; I could see the Captain was having to hold significant pressure to maintain level flight so we requested a block altitude 5;000-6;000. We ultimately requested priority handling with ATC and decided the best action was to request vectors and go to ZZZ1 instead. We coordinated and communicated with dispatch; the flight attendants and passengers our intent to land at ZZZ. As we conducted the approach; through normal procedural habits and discipline; we extended the flaps beyond the QRH recommended flap 20 setting. Once we realized our mistake (the flaps were to be at 20 per the QRH); we simply retracted the flaps to 20 where they were supposed to be. This happened fairly early on the approach and didn't effect the stable approach criterion. Our decision not to reselect stabilizer trim after disconnecting was due to the QRH not directing us to do so. Also; the aircraft was in a flyable state; albeit more difficult than with a functioning autopilot and stabilizer trim.Uncommanded; excessive and undesirable stabilizer trim output for state of aircraft automation and configuration.
CRJ-700 First Officer reported a runaway stabilizer trim system; causing a diversion.