Try using a set of big wheel chocks, at the front and rear of both front wheels to keep the truck from going anywhere, and see if you can duplicate the noise on a paved surface such as a driveway or parking lot, with someone watching the suspension from a safe distance to the sides as you try to move the truck forward and back. We found the source of a bad shudder coming from a motorhome that way- - - -the surge brakes of the 2-wheel tow dolly being used to haul a small car behind it were locking up repeatedly during a stop due to a broken damper spring. It felt like the rear end on the motor home was about to come apart every time the brakes were applied. We got the rig home several hundred miles by opening the bleeder valves on the tow dolly to disable the surge brakes, and planning stops carefully to accommodate the reduced braking ability.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!