I had a similar issue a there is a lot of good advice here, check: tires, wheel balance, drive shaft balance and pinion angle, quality shocks and correct shock angles for lowered trucks. Do you have polyurethane body bushings? They will make for a "choppier" ride. I chased a vibration for a couple of years and addressed or checked all of these components. Come to find it out it was a new set of Moser axles that were machined incorrectly. Go figure?
Oh well, at least I have it figured out now.