First I would yank out all of the stuff you have stored in your rear compartments (jack and whatever else). Then take it for a spin.
Next recheck your shocks for leaks or if they are loose.
Finally, the last place to check is the rubber gromets in your leaf springs. Check my FAQ below for links to places that sell poly bushings. Now that I think of it, I bet this is the cause of the problem.
-Ryan