If you slowly increase the engine RPMs with the truck parked, you should create the growl/vibration when you match the 50 mph engine speed. That would be around 3000, 3200 rpm. Watching and listening to the engine may give you a clue.
Possibilities:
Vibration damper, flywheel, pressure plate.
More obscure, iron pistons with a crank balanced for aluminum or mixed aluminum and iron pistons, one spark plug misfiring under load.
