1992 Chevy Suburban Hard shifting

When i shift into reverse, primarily when vehicle facing downhill, I hear a loud clunk. I suspect the transmision. So i had 2 diff friends listen to the clunk. They hear it from the rear end of the vehicle.So I replaced the U joints in hopes. That didnt do anything to help. Still shifts hard. But sound comes from the rear of the vehicle. Any help would be greatly aprreciated. How do I fix/check a rear end or tell if its the trans???