I just purchased a 2011 Yukon Denali with 22'' wheels. I have 1034 miles on the vehicle now( half of those miles are from the dealer test driving). I first had excessive vibrations in the steering wheel around 50mph. Took it to the dealer and they "fixed" it. First trip on the freeway the vibrations started again at 65mph. Took it back to the dealer. They couldn't balance the tires and wheels so they replaced all four tires and wheels. That seemed to have worked. Now I have a "thud" or what feels like a "pressing" vibration in the seat. I can feel it when i let off the gas around 35mph and coast. I drove with my dealer this morning and he felt it as well as one of his tech's, who also drove it. They said they have no idea what it is and they will contact GM to try and fix. Anybody else have this problem?