1996 Pontiac Sunfire Squatting and jumping

As part of normal wear indicated, I changed front brake pads and rear shoes. I did not turn rotors or drums because they looked and felt good. Ten miles later the rear of the car began squatting and jumping when I hit the brakes. The local garage in town said that I had one set of shoes swapped wrong, front to back, they corrected this, but 10 miles later I was back again. I took it to a GM dealership and they determined that the drums were warpped, so they changed them and said that the shoes were in good shape. 10 miles later it was back again. When I hit the brakes, the car seems to squat and it seems like the car is jumping with each revolution of the wheel. It is definately coming from the rear of the car. I don't want to let any more mechanics guessing as they take my money.