Tires: Tire problem, bad tire, tire problem

Question
I have a 2002 Tahoe that I just bought and I have noticed that it shakes at highway speeds, (from 65 on up) I have taken it to a tire dealer they have balanced the tires and it is still doing it so they said it was a tire in the back that was bad you can feel it more in the seats then in the steering wheel and below 65 it drive smooth as can be. Can you please help me firgure out what is going on since I just bought this truck and I hoping that it is just a bad tire and not something more! PLease Help!!
Thank You,
Justin J

Answer
If the tires were properly balanced and you still have the vibration then the next item would be to check the wheels.  You could have a bent wheel.  Also, depending on the age of the tires and the remaining tread depth, it is possible that a tire beginning to separate.

If your wheels are OK then you may have a mechanical that is best diagnosed by a qualified mechanic.