Q:

How to fix rear differential leak on 1974 Ford F250

I have a 1974 Ford F250 RWD that I use very rarely. I take it around the block once a month to make sure it still works.
A while ago I noticed that the rear differential was leaking from (what appeared to be) the cover gasket. I replaced the gasket and it's still leaking. It now seems that the differential is leaking up where the driveshaft enters the differential.
What is likely to be the problem here? I couldn't find any relevant info in my repair manual.

A:

The pinion seal is most likely bad.  Assuming this is a Ford 9" rear end...
You'll need to remove the driveshaft yoke to replace it (the piece your driveshaft bolts to on the third member).  You shouldn't have to remove the third member to replace the seal either.  
Just remember when you reinstall the nut adjusts the preload on the pinion, so you can't just torque that nut down tight and call it a day.  I believe the preload torque is measured in inch-lbs, so obviously you don't want to attack it w/ a gigantic cheater bar or 1/2" drive impact.
If you're familiar with the concept of installing wheel bearings you will know what I'm talking about in regards to bearing preload.
Good luck!

