I have a 2015 Tahoe LTZ that rides rough. It has 4,800 miles and 20" wheels. You feel every bump in the road and it feels like it's coming from the rear. So far he dealer has replaced multiple tires, the drive shaft and the rear differential. It seems as if the shocks are very stiff. Is this just my imagination, is there a suspension issue or is it a very stiff truck?