Truck Lift/Airbag Question

I've got a 2003 Ford F350 4x4 Crew Cab. It's got a six-inch lift in front and a four-inch lift in the rear (blocks). I do not like how the rear sits lower than the front and I want to level it. At the same time, I'm told that I should get air bags to keep the rear from sagging when I tow.

I don't know much about this lift stuff, so I have a few questions. Do airbags actually lift a truck or are they something separate that would be added to a lift for leveling purposes? Any recommendations for getting my truck level and curing the sagging issue when I'm towing? It actually just might look like it's sagging when I'm towing because of the unlevel lift. I would think that since it's a 1-ton, there shouldn't be much sagging when I hook up just a boat.