'98 Eldorado

I have a '98 Cadillac Eldorado. There is vibration between 55mph and 75mph. I feel it in my legs and a little in the steering wheel. I have new Michelin Harmony tires that have been balanced twice. Took it to the mechanic and he doesn’t know what it is. Any ideas?

Rotate the tires front to rear and see if the vibration changes.

Already tried that. Thanks though.

Your tried it and… What if anything happened?