[bobcowan]

Lot of things besides the tires can cause a vibration. You've already checked and balanced the tires more than once - time to move on.

Get the car up to about 65mph. Put it in neutral and let it slowly coast down below the vibration zone. if the vibration changes, it's most likely the drive line. If it doesn't change, it's suspension.

Suspension problems could be a loose tie rod end, ball joint, etc. Check everything very carefully.