rear control arm bushings
there are 8 bushings , 2 upper and 4 lower and the 2bushings in the rear axle ears
replace them with oe style or energy suspension (they have a kit just for the rear)
or you could go with aftermarket arms which will have the bushings already installed, and dont forget the 2 bushings in the axle ears (hrpartsnstuff sells a tool to change those)
probably could use new shocks as well