sounds like shocks to me... mine started doing it after i put in the rough country shocks. i guess they are a little stiffer or somethin'. if i let off the gas when i hit a bump it just bounces once- try that

 

When I think about trucks I think diesel primarily... I know they don't differ too much when comes down to it but I think about where I've seen the applications.

And if I may comment on your picture: I haven't seen them mounted to leaf springs before. We always fabricated custom boxed brackets and welded them to the axle and frame. Pretty neat.

 

My truck was jumping around some at highway speeds and I found the left rear shock had lost it's action and was causing the odd feeling. It sneaks up on you so it isn't always easy to diagnose. Shocks cured the tracking problem, Caltracs cured the axle hop.

 

Thanks for all the input. I'll take a closer look at the shocks and go from there.