AH64ID is correct in that the Hensley is a band-aid and an expensive one to boot. A properly loaded trailer, properly set up WD hitch, tires aired correctly and a properly sized tow vehicle a trailer will not sway under normal conditions. But Mr. Murphy is always on the look out for an opportunity to cause problems. You catch a truck bow wake at the right moment or a cross wind over a bridge and it will start a sway that you may not be able to recover from unless you have a sway control device.
A sway control device is worth it's weight in gold when you need it.
But the Hensley will only mask the problem which you need to correct. There have been many suggestions to check your weight, hitch set up, tires, etc. Even with all of this set up correctly, you are pushing your little truck towards it upper limits.
If you were close enough, I'd be more than happy to try and help you on this issue. Do you happen to be close to Monroe, LA?
Ken