When I put truck in 4 wheel drive and start to back up, my rear tire locks up. I have had several mechanics look at it and they don't know what the problem is. Works fine when not in 4 wheel drive and works fine going forward while in 4 wheel drive.

 

Do you have a aftermarket locker? might be a good idea to pop the rear diff cover off and check for any severe wear.

 

No - it is original that came on truck

 

Did pop cover off - there was no sign of wear

 

On my ol' Ford F-250, my left rear tire would lock up. Only going forward. But the issue was the Kingpin kept sliding out, and it was obvious when the cover was pulled. Sooo I am at a loss, Sorry I can't help ya further... Someone will though.