You have bad shocks-and- or tires that need replaced-while car is setting try to push down on the rear and see if it goes down easily and then bounces when released-jack rear end up-try to spin tires and see if they spin freely-check tires for cupping or funny wear patterns-you also may have brake shoe that are coming apart and locking a wheel up without putting on the brakes-so lots of options here-hope you get it solved-cheers Denny