Bad vibrations

Chevrolet/GMC has had this issue for many years, it is due to the solid frames they use.  Many notice the passenger seat shaking while driving alone.  In the late 2000's they recommended new body cab mounts that were softer, then in the later 2010's tire balancing was crucial.   Most found that having your tires Road Force Balanced was the key to fixing the issue versus normal spin balance machines. Road force balancing places pressure on the tire (like a road surface) and better recreates what many are feeling when tires are balanced as well.  Most GM dealers have Road Force machines in their service bays because of this, but many never go to their dealer for tire service.