I agree with the comments above. Probably a combination of load and weight balance in the trailer and hitch adjustments.
If you make those adjustments and still have an issue check the tires on your truck. I once had a tow vehicle that had too soft of side wall on the tires, once I switched to a heavier load rating tire that helped to cure some sway issues as well.
Tom and Lori
A dog named Trixie
1996 F250, 7.3L, TS chip, 4" exhaust, EBP delete
2012 Cruiser 335SS