I just had a mechanic put in a used rebuilt set of gears in the rear end of my 1991 GMC 1/2 ton 4×4 truck. The problem we are having is when you drive it, it starts out fine then there is a jolt and it acts like it wants to go in revere.
He brought it back to his shop and picked it up and spun the wheels, they seemed fine and then Wham, loud noise and the wheels stop and wanted to go in revere. He is stumped he has never came across this before what could of happened?
Is the Driveshaft disconnected when spinning the wheels up in the air?
Does the rear end have a Locker Unit? Limited Slip? Or Posi-traction?
Sounds like something has backed off and is periodically catching. Remove Drive Shaft. Drain rear end fluid and remove cover. spin and inspect the work. Make sure the axle clips are not backing out or damaged.
1991 GMC K1500 Rear End Cover