During the initial "gas pedal push" during acceleration and after the truck rear-end settles after a hard stop. It does not do it during deceleration (except as noted before), does not do it on bumpy roads, nor around corners, nor when it shifts.
It is 100% related to the truck body shifting on the suspension, as i have been hunting this issue down for 2 years now, and their is was no difference from before and after the noise started as far as driving differences. It actually started about 1 1/2 to 2 years after i installed the lift kit, and if you would of seen the rubber upper mount bushings that i removed, you would understand why i thought it was the shocks.
matter of fact, if you slow down to a crawl before stopping, it doesn't make the thumping noise during the initial acceleration, it is just after normal to heavy braking stop.
The shock piston rods going into the upper mounts are at a steeper angle than factory, and the mount bushings do not make a very good "squish" to the mount when tightened down, hence why I am thinking it may be shock related. I'll try and get some pictures tomorrow to post.
The only other thing i have not done, is go back around and tighten up the suspension components in the rear (i've been lazy about it).
02 Screw FX4
Jensen VM9512, nav101, xm radio, SVT Lightning tail lights, FX-R HID Projector retrofitted SVT lightning headlights, Fabtech 6" lift w/ steering stabilizer, Bilstein 5100's, Yukon 4.56's, ARE Bedcap, ProComp 285/70/17 AT's