I've got a vibration that starts at 55mph and it will max out at 65mph, at 66mph it quits. This is a 2500HD 4X4, 6.0L gas. The transfer casing went out at 86K, had it changed out. Went out again 5K later, said I had a bent drive line, changed it out. Now the vibration is bad. The company that sold me the transfer casing now claims it is not the transfer casing causing the problem. Wheel bearings check out, new drive line. Got any ideas?