If your rear brakes are the drum type I would be curious if the ebrake is hanging up. We see this a lot on the tacomas and tundras here in MN. When they stick the make a horendous vibration after they warm up. Usually you can tell that the rear brakes are hot when this happens as well.
The other thing I would look at is how the front wheels are rotating. With the vehicle on a host start it and put it into drive and watch the front wheels. If they wobble, stop the car take the wheels off, secure the rotors with the lug nuts, start the car and let it spin in drive. If you can still see a wobble in the hub than the hub is bent. Not uncommon for this to happen even on front wheel drive vehicles.
Lastly have they tried rotating the tires. Its possible that one of the tires on the front is starting to seperate and may not be obvoius at this point. Rotating the tires will either make the vibration worse or better depending on what axle the bad tires ends up on.