If you know the tire/rim combination is not out of round and is balanced it could be the driveshaft.

When this occurs are you coasting, accelerating or maintaining speed under load?

BTW, if tire balancing was done with weights instead of Equal or Centrimatics are you sure they are still in balance? Jim Keller had a demonstration proving his tires rotated on the wheels after being installed under heavy braking so do not assume because someone added weights that the balance has been maintained.