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It may not be the rear end. It may be your U joint where it meets the differential. Does it only do it in reverse or does it do it again when you put it back in drive?
Dak
with the wheels blocked place the trans in neutral and check for any play between the driveshaft and yokes.
also check for excessive backlash in the rear end yoke you shouldn't be able to turn the yoke more than 1/4 rotation (there will be some normal slop in the spider gears the actual backlash should only be about 7tho of a inch)