First, make sure the heaviest part of the load sits over the trailer axles. If the distance between the rear truck wheels and the trailer wheels is greater than the distance between the front and rear wheels of the truck it will sway. This was the case when we pulled a horse trailer with a Bronco. We fixed this problem by installing an anti-sway bar between the truck and the trailer. It was a long time ago but I don't think it was very costly. Maybe a couple of hundred dollars. Good luck!