If your rear end seems a little more tumultuous than it used to, you might try changing your diet or taking a little Bean-O before meals. If that doesn't do it, you might need to replace your rear suspension bushings. Your shocks suspend your car softly in the rear, but the points at which the other supports attach to your car's frame in the rear have their own little shocks called bushings. These rubber bushings can wear out, and then they become much less squishy. This can cause your rear end to thump or creak.
It's not too difficult to replace your rear suspension bushings. Follow the steps and you shouldn't have much trouble.