These trucks are famous for driveline vibration at highway speed. I worked at the dealer when these trucks were new and saw quite a few with this issue. Most of the time it was the driveshaft itself that was the issue. Could be either front or rear. Mark and Remove the front driveshaft and take it for a short test drive. If the issue is gone, reinstall it and clock both ends of the shaft from the original position. While it’s out inspect the u joints and lube the shaft bearing that no one ever seems to do. Inspect it for slop and binding. Also I see several bent or deformed rear driveshafts after u joint replacement. People usually heat the caps release the plastic that retains the caps, then while they are still hot, beat out the old u joint causing shaft damage.