Unless your center pins are broken, or u bolts loose, the only other movement is in the shackles and bushings. just outa curiosity have you checked he back space on the rims ?
I agree, jack it up until rear is hanging, see if it moves back, if not push it back while jacked up by frame , no weight on tires, when pushing back see what moves, may just be bushings need replacement, while jacked up also check u bolts, wouldnt hurt to lossen them and check center pins as well.