Noise Sounds Like Dragging Muffler

I have a 2001 Dakota and it is making a noise from behind the cab
somewhere that sounds like I'm dragging my muffler (metal on the road
sound) or something rattling under the bed. Nothing is dragging back
there though. The sound gets faster asI drive faster and happens
mostly as I accelerate or hit a bump. Applying the brakes or putting
the transmission in neutral does not stop the sound. The sound is not
consistent either. I took the drums off the rear wheels yesterday and
nothing came out that would indicate an object rolling around in the
brakes. I also tried to shake or turn the prop shaft and that was very
solid.

Any suggestions?

--
Jerry