I towed my trailer with an Expedition and it was not an ideal TV for a heavy trailer like he has. For one thing they have an independent rear suspension so they tend to lean to each side a little more than a solid axle. Mostly though he needs new shocks. A set of bilstiens would help a bunch.
His hitch is set up well. If he distributes more weight to the front axle the rear will get moved around more by passing vehicles.
Measure the frame of the trailer to make sure it is level from front to back when WD is hooked up. Slightly nose down is OK but not nose up. Should be within an inch and a half from front of trailer to rear.