Run a floor jack under the pumpkin and raise the tires off the ground a couple of inches. Chock the front wheels or run the nerf bar up against a big tree. Run a floor jack under the pumpkin and raise the tires off the ground a couple of inches. Run it up in gear to where you feel the vibration and note the speed for sure. Remove the tires and run it up again. If it still vibrates, it's not the tires. It's in the driveshaft, trans or motor.