I just purchased a 2005 Avalanche Z-71. It has after market 20" wheels and Nitto 305X50's on it. The truck rides very smooth until I get to 65 then I get a significant vibration. The dealer said they's take care of it and they have had it at the shop today for 4 hours, balanced the tires twice and are saying that it still vibrates over 65. They said they need to place some "rings: between the wheel and the hub to take care of the vobration. Has anyone heard of that before? I've had several trucks with oversize wheels/tires and they were harder than usual to get them balanced but they always did.