Get a load-distributing hitch and an anti-sway bar. I have used them on my enclosed trailer for years. The load-distributing hitch transfers weight to the front of the truck, and the anti-sway bar absolutely eliminates any side-to-side movement.

Hitch - http://www.etrailer.com/Weight-Distr...e/RP66542.html

Anti-sway bar as Paul mentioned in the previous post.