I'll second a little of what
@MrFancypants said. With a bumper pull trailer in that weight range, a rear swaybar and/or airbags is going to do very little if you are getting trailer sway. If it was a 5th or bed camper then that's when you'd benefit from them, with more weight over the bed.
I'll ask - are you actually getting trailer sway or does it just feel unsteady? It makes a difference. If it's just unsteady feeling in the truck, then I'd double check things like tire, alignment, tire pressure, is the truck lifted or leveled? The stock duratracs are known to be a bit squirmy by nature. The one other thing that comes to mind is wind. An enclosed trailers catch a lot of wind.
I spent last summer towing a 14k actual weight equipment trailer around. I had zero issues and truck was rock solid pulling it. Not sure the exact tongue weight, but was getting about 3" of squat on the rear which made my truck is perfectly level. And I had the weight centered just slightly in front of the trailer axles.