Rear end bouncing bouncing between 5 and 60 MPH

[Jstuart83]

Good afternoon

Ive got quite an issue I need some external input on. 

Ive spent countless hours on the internet trying to figure this problem out and spent way too much money.  I’m starting to think I’m a little crazy.  

I have an 05 GMC serria 1500 ext cab 4x4 with a 6 inch RCX suspension and a 3 inch body lift.   The truck has always had the bounce/vibration since I purchased it stock.   I assumed it was tires or wheels.  So In the last 5 years I have replaced:

Tires multiple times it seemed better with 35 nitto mud grapplers, but I got tired of the noise.  Maybe because of the weight?

Wheels.

Gears to 4.88.

Axles.

Drums.

Shocks.

The shop that did the gears was suppose to check the driveshaft balance and they said they changed out one bad u joint.  The balance was good according to them.

I put the new style rear cab mounts that were suppose to address the frame beaming.

I recently have been thinking the second set of drums I recieved were warped or not balanced properly which might explain the bounce/vibration at higher speeds, but I don’t think I would feel that at a lower speed like 5mph.   Also I don’t think I would feel frame beaming at a lower speed.  At the lower speeds it feels more like a bad tire.  I’ve rotated the front tires to the back and still have the same issue.  Any opinions or comments are welcome.  I’m not a mechanic but I know enough to be dangerous.  I’m starting to think this could be a transmission issue some how.