[Dick M]

You might have a broken or missing rear center "square head" spring bolt. The spring assembly can and will shift in the cross member without the square bold head inserted into the cross member. Try removing the four U-bolt nuts, drop the rear end enough to see if this is your problem. It can be difficult to get the square head bolt re-seated into the cross member. I have found the easiest way to get the square head bolt inserted into the cross member is to put the spring assembly into the cross member, place a jack stand under each side of the rear axle, and then with the weight of the car resting on the springs rock the rear of the car until it drops into place.

Another cause of your problem could be badly worn spring shackle bolts and bushings.