rear end rattle

[shrike]

is driving me nuts! (no pun intended)

I have been hearing this noise since I bought the truck, and I think that it is getting a little worse. Its hard to tell since I drive it every day and if it is getting worse it has been pretty gradual.

It only rattles when I hit a bump or a dip in the road. I dont think its a drivetrain issue because it doesnt matter if I am in gear or not, accelerating/decelerating etc.
I have finally narrowed it down to something on the rear of the truck (by driving over a speed bump).
My initial thought was the tailgate, but im not sure because if I go back and shake the hell out of the tailgate it doesnt make a sound.

The sound itself is kind of a cross between a clunk and a rattle. when I first got the truck I thought the bed was loose, but that is not the case. I have been under the truck many times and have even had the guys at the brake shop look for it but nothing is obviously loose. New shocks on the rear didnt make a difference either.

Any ideas? Anyone had an issue like this?