If you take it to a frame shop they will put the car on a rack and make measurements and be able to tell you exactly what the problem is. My guess its bent control arms due to poly bushings. Poly bushings are a really bad idea on the rear of A body cars. I found out the hard way. They destroyed my upper control arms. The A body rear suspension is designed for the give of rubber bushings as the suspension is compressed. The poly bushings don't give so something else has to.
Here is some good info
https://www.umiperformance.com/home/...pros-and-cons/