Air Lift 1000 air springs.... installed on my last two trucks. Definately get individual fill fittings for each bag. I have my air fill fittings routed to become my upper rear license plate bolts, and it looks trick. It basically makes the rear of your truck into an adjustable anti-sway bar, towing or not, I love mine. $300+/- installed.
Firestone Ride-Right Air Bags are also used by a few members. Do a search on Air Bags, there is lots of info.