As others have already said, most likely it's the bushings.
Next most likely culprit is the sway bar links. They have ball joints on each end that can develop some slop. Unbolt the link on one side and check them for slop. It's hard to detect without unbolting one side because there's almost always a little bit of twist being applied.
Lastly, it could be the rear lateral arm on the suspension. There are two arms that run from the center out to the wheel. The rear one has a turnbuckle in it to adjust toe in/out. On the outer end there's a ball joint that can develop slop. To check it, jack up the rear and grab the tire at 9:00 and 3:00, then try to rock it by pushing on one side and pulling on the other in alternating fashion. If you detect any slop at all, it's bad.