Car vibration at freeway speeds; why?

Can you touch the floor board and feel the vibration ?
If not you have air vibrating something, set on jack stands and safely
supporting the rear tire of the ground see if you get the vibration.
if you do it could be as simple as a wheel/tire, or drive shaft is out of balance that will take your tail shaft seal out, check your pinion for play, but I would figure the mechanic checked that.
Air is a funny thing sometimes, ever herd a emergency brake cable rattle and vibrate . I have, but I think it was in a truck.