I have a 2010 Yukon Denali with 81,000 miles on it. At about 75,000 miles it started to shake at 50 and at 80-90mph. It is relatively smooth at other speeds. This is a shake that is felt in the passenger seat if I reach over and put my had on it and feels like a pounding of two or three beats per second. Not really in the steering wheel at all but in the seat and dash. I have Michelin MS/2's and have hd them since about 65,000 miles. The truck use to be as smooth as silk at all speed to over 100 mph. The dealer has road forced balanced and says the wheels and tires are perfect. I also had them check the drive line. They cannot find a thing. What else do I have them check? I want it back to the smooth ride I once had.