1999 Oldsmobile Aurora Vibration at highway speeds

Purchase this Aurora new and have always noticed a slight vibration around 65-80 mph that is getting progressively more noticeable, to the point that I cannot stand driving on the highway. It is not tires, tie-rods or suspension as they have all been checked. I suspect the issue is in the transmission output shafts that are wobbling, or a bad engine mount. You cannot feel in the steering wheel, but in the front of the car itself. Half shafts have been replaced! Any ideas on how to fix or repair at reasonable price?