75mph vibration

If you're still having trouble after you get the tires checked again, put the rear on jackstands and "drive" it in the garage. If it still vibrates, then you've ruled out the front end as the source. To rule out the wheels as the source, remove the wheels and drums (put a block under the brake pedal) and try it again (you'll have to start the engine in neutral so you don't need to shift through reverse). If it still vibrates, then the problem is in the drivetrain somewhere. I assume you've already tried revving the engine while parked to rule out engine vibration.