Hey thanks for all the responses. I'm checking your suggestions one by one and so far the only thing that I could find that might be it are the sway bar links that have play in the bushings. I removed the sway bar and links and doggone it...Still thumping.

I need to borrow someones wife or girl friend to drive the truck while I sit in the truck bed and listen for the thump. OOPS! I DIDN"T SAY THAT. WHERE'S MY ERASER?

 

Found it today. The rubber bumpers on the rear bed box pillars that fit against the tailgate when its closed.

Silicone spray and all is well !!!

 

so who's wife or girlfriend did you borrow lol

 

nice at least it was an easy fix.

 

Gal that drove a bulldozer for the county. She had a good personality.