Drivetrain vibration

You need to figure out if its engine speed related, driveshaft speed related or wheel speed related. An engine vibration will change with engine speed, regardless of gear. A driveline vibration will be fast in your case 4 times faster than the engine speed or more due to the 4:10 gears and the OD. Driveline vibrations also quite often change with load. Wheel speed vibrations are slower and sometimes change when loaded differently, like changing lanes fast.

Garth