If its only the rear acting up then take a look at 2 things. Firt make sure the grub screws are in place for the 2 drive cups in the rear, one on the tranny and the other on the diff. If these are in place then you have to open up the diff and take a look there, it could be either the outer 13t and 43t gears or the inner spider gears.

If you dont have any alloy diff cups then you should get some to avoid this kind of problem in the future.