I would check torque on all the driveline bolts. I recently bought a 300tdt and on the 3 hour trip home it would shake only under load. The next morning, on the way to a friends shop, the shaking go so bad I Pulled over and shimmed under the car. Two of the 3 driveline bolts had fallen out of the rear diff. and the third was barely hanging in there. I had to drive the car about a mile to a local muffler shop. They put it on the lift and two bolts and a $20 tip later, she drove smooth as the day she came off the lot.