Auto Vibration a higher speeds

I own a 2004 Cadillac Deville, purchased with a certified warranty. The car begins to vibrate at 58mph and gets worse at 65mph. At 72mph, the vibration decreases. You can feel it throughout the front seating. I have 12,000 miles and it is a V8. I had new Michelin tires installed today, replacing the ones on the car. The Caddy still vibrates at the same speeds. No change. The tire dealer thinks it is caused by the wheels or CV. I am prepared to employ an attorney to get it repaired. I have only owned it for 2 months, and it has vibrate since the purchase at the higher speed listed. Do you have any advice. The car is fine except for the discomfort on the highway. Thank you. :Cry: