What year car?
High speed balance on the tires/rims? Try installing your old tires and rims to see if this changes anything. Had a buddy buy new American Racing rims and one was incorrectly welded putting it out of round. Have you checked the rims with a dial indicator to make sure they are not out of round or bent?
Out of round or "heavy" tire? This can happen and give you a vibration as well as a shift of the radial belt inside the tire.
Bad bearings at the axle or pinion. Have you checked the axle bearings? How much play in the driveshaft if you grab it and twist it left and right?
Front end aligned? Could be out of alignment or worn bushings/front end causing the alignment to change with higher speeds.
Bad front wheel bearings or too much end play. Check for this.
Might be the pinion/driveshaft angle, worn out rear control arm bushings.
Did you "jack" the car up in the rear or use bigger rims & tires? This will affect driveshaft/pinion angles.
Is the new trans mount centered?
It could still be an engine mount problem, but they should have been the same in appearance other than one being left and one being right.
Many things to check & look at other than engine mounts which could be your issue.