1964 Chevy Corvette binding

when I turn to the left or right there's a binding similar to the front wheels of a 4 wheel drive unit when in 4 wheel drive.it feels like the whole rear of the car is trying to hop. I changed the oil in the rear end(posi trac) to the proper spects. but Its still there. Maybe something else but the u joints seem tight and the swing arms seem tight no play. I like to find if its the differential but we don't have a lot of old corvette shops in town. The transmssion/diff shops ,in my experience, go right to the posi or diffential.