My 11 crew cab sport 4x4 no lift kit, vibrated in the rear at 70. Sold not forgotten.
My 16 crew cab Laramie 4x4, no lift kit vibrates in the rear at 70.
Multiple sets of wheels, factory and aftermarket, Multiple sets of tires over 230k miles combined.
Drive 65, no vibration, drive 75 no vibration. Drive 70 vibration. Got to be in the design of the vehicle. Probably drivekine related.
My 14 RT, does not vibrate at any speed. Aftermarket 5 inch driveshaft.
I'm thinking and it's a hunch, it might be driveline related. Maybe Carrier bearing, maybe driveshaft, could we control arm bushings, etc.
Last, doesn't seem to reveal itself if loaded or towing.
My thoughts