At that length of trailer, a wdh with integrated sway control is recommended. This would be along the lines of reese dual cam, Reese strait line or equal-i-zer. Higher quality and a lot more expensive is the propride or the Hensley. You already have a wdh so you could try a cheaper solution of putting 2 friction sway bars on but again, the above solutions are more recommended.
Also lets address the sway issue. Was it true sway (increasing oscillations from side to side), a bow wave affect or trailer wagging the truck or porpoising? If it was truly sway, you should not have sway to begin with, these systems don't prevent sway, they help control it if it starts. The key is to load properly to prevent it in the first place. Loading too light on the tongue and heavy in the rear is likely the cause here. Try moving more weight forward of the axles. Also double check your wdh set up. If it was one of the other issues I mentioned we need to dial your hitch in.