We encountered severe "chucking" with our 5er from the very beginning. The push/pull shaking occurred during all aspects of towing. Uphill, downhill, level ground, mattered not. In trying to find a remedy, I researched everything I could find. I decided to install a "5th Airborne Kingpin" That helped dampen the chucking somewhat but it was still very noticeable. Next I installed Firestone Air Bags on the truck. Finally we had a 5er that was pleasant to tow. If I had it to do over, I would install the Air Bags on the truck first. That might have saved me the price of an air ride king pin.
Would like to hear what your friend finds and how he fixed it...