As noted above, a good WD hitch would solve the sagging, and has other advantages that different springs or air springs could never match, due to simple physics.
Also, weight can certainly be an issue, but one notable drawback with an enclosed trailer is the parachute effect. It wouldn't matter if the trailer only weighs 1,000 lbs., the mileage will take a dump. And the near mandatory side door alone adds about half of that weight.
With a short and light tow vehicle like a Durango, any side wind (natural or from passing tractor trailers) can make it a handful. A WD hitch with active anti sway will help, but the laws of physics still wouldn't be on your side.