There is a knocking sound coming from the rear of my 4Runner whenever I hit bumps. It usually doesn't do it in wet weather. What the hell is it?

The cheap answer is there may be a loose attachment for a shock absorber or a sway bar (noise could be from anything attached to both the suspension and the chassis). If you are lucky, you can see the problem. You might be able to hear it if you are under the car and someone bounces the car up and down while you listen.