Vibration ONLY at 75 mph

I have read and re-read your post.When I mentioned Harmonics it was not just about the engine.That was a example. Anything with movement can cause vibration. Loose siding flapping in a wind storm for one.Any imbalance in motion will cause a vibration be it rotational or directional. We understand your frustration and are only trying to "think" of things that may help.You got a tough one and I wish you the best at trying to find and fix the problem.Be thankful that you haven't had comments like "Don't drive it at 75 and it won't vibrate ya no mo". Again We are only trying to help.