It is entirely possible that you have bent hubs/axles on the rear from the beating on them .

 

I'd say its a balance issue, espically since you can time it based on the speed of the truck which means its cause is the Wheel/Tire and not the road.

 

I did that, there was 1 rim a little bent. I put on another rim.

 

why not pull the frt drive shaft , process of elimination . I would not be surprised to know either the frt or rear was out of balance

Check the motor and trans mounts

 

I dont see how this is possible. But I bet you warped the rim

 

Why would it not be possible? Depending on how hard they were beating and how much heat was applied. I'd raise it, remove the wheels and drums. Start the truck and put it in first and see if there is any noticeable wobble in the end of the axle. You might also notice that the drum was scraping on the back plate at first.