Sounds like one or both struts might be blown out. When the rear struts went out on my wife's Maxima, the rear was all over the place because the struts weren't keeping the wheels planted firmly on the ground.

When you push down on the rear of the car really hard, does it bounce back up realy fast or does it still bounce up an down a few times kind of slowly? If it's all mushy feeling, then the struts are no good. Hope this helps...:shrug