:M
This is very poor advice.
You can not "bandaid" this via WD or sway bars.
A properly loaded trailer DOES NOT sway.
If you rely on WD bandaid to fix this what happens when you need to make an emergency maneuver? You "fix" will ultimately not be able to cover up the problem.
First you need to WEIGH the trailer, get tongue weight along with the weight on the axles. The tongue weight MUST be at least 10% of the axle weight. Ideally it should be 13%-15% for a stabil tow.
To me it sounds like too little tongue weight but that is only a guess. Some trailers may be light on the tongue and when loading you must make sure you add more weight in front of the axles.
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Dead solid advice! Fix the wag (likely more hitch weight/less after axle weight), THEN add the sway control!