So I live in NH and just took my MY04 S out of winter storage. I bled the brakes, did the oil, tranny, and diff. I also had 2 new rears mounted, but kept the fronts. The front right brake disc was seized so I wacked it with a rubber mallet to free it while I was checking all the pads and calipers.

Problem: At 72MPH and above the car shakes violently to the sides. What have I done?

 

Was the car on jackstands? If not, it sounds like the front tires are flat spotted. See if someone in your area will let you put their fronts on your car to test the theory.

 

I did have the car on jack stands. I also had the wheels/tires indoors with climate control and out of the elements.

 

Sounds like tire imbalance but given they were not under the car check the rotors for lot rot.....AKA that crappy spot of oxidation that grows under the brake pad and literally swells the rotor and no matter how hard you brake doesn't go away. But still it sounds like tire imbalance. My .02 cents

 

Check your lug nuts ... Loose wheels and your car will be dancing