91 Camaro clunking in rear end

How many miles on the u-joints? And have you changed the lube in the differential, its suppost to be changed every 35000 to 45000 miles. Most people don't do this service as needed. Resulting in failure. Good luck

I put new u joints on it. Same problem. Added fluid. Same problem.

Change the rear end fluid is what I was getting at you might find chunks! A second thought is maybe the suspension arms bushings, where they attach to the rear diff. And/or the frame. Good luck