Vibration ONLY at 75 mph

Is it possible that you have an air problem? That is, at 75 MPH the air traveling over/under/around the car is buffeting and setting up a vibration through the whole car. Try changing the stance of the car, raise or lower the front and see if the vibration changes or disappears. I'm just trying to think outside the box, because it looks like you have checked or changed most everything else.

Jack