Prius (2010-12) :: Tire Noise And Shaking Felt In The Floor

So at 19+K I notice the car has tire noise and shakes. Take car in for 20000 mile service and the dealer tells me I have cupped tires. They say the car must be out of alignment to get cupped tires. I think not because the car goes straight down the road. They have nothing to offer in regards to getting new tires free or adjustment on new. I should have pushed hard for a resolution to the problem right then and there but did not. I rack up miles fast so I put up with the problem and at 37000 have the dealer put on new tires and check the alignment. Alignment was in specs but not to complete center. So I have them align the car.

On the way home I notice I still have a shake, felt in the floor and the passenger side head rest shakes back and forth. They rebalance the tires and tell me all fixed. I drive off only to feel the same shake.

I go back and insist on talking with the service manager. Take him for a ride and he can't feel the problem and says all he feels is the normal Prius road roughness. He says if I leave the car they will check it out further the next day. I tell him maybe the reason my first set of tires went bad is due to the shake. Look further than the tires! Next day when I pick up the car they tell me they road forced balance the tires and let air out of the tires to soften the ride.

Of course the car still shakes. I take the car to a different dealer and tell them I need the car completely checked out and not to focus just on the tires. They tell me the tires where way under inflated and out of balance, one even by one and a half ounce. They tell me all is fine. I drive off and still it shakes. Go back and take the tech for a ride. He can't feel the problem.

So got a shake in the floor that is best felt at between 65 -72 miles per hour? How do I correct this? If this is normal for a Prius why did I not feel it for the first 19000 miles? I say it must be drive train related but I can't seem to get people to not focus on the tires. Car is 15 months old and I closing in on 40,000.