I have though long and hard about the same thing. I picked Jake's brain for a few weeks. I have more projects than time and money right now as it is. I might build a system like my Brother in law has on his F-450 though. It retains the rear springs as it then removes the spring hanger and mounts the shackle to a pivot point back near the hitch. The pivot arm is then suspended with an air bag. This doesn't give more load ability but it will provide a better ride and load leveling. The only other thing it would still need is a track bar.

Also these systems (the talon and primary) would be pretty easy to build as well. Not much for off road but towing and daily driving I bet they would be nice.
http://www.stengelbros.com/AirRideSuspensions.htm

 

To go along with my last post here are two options that kelderman offers. The front they do just like the question here. They add a bag and cut the bottom spring to 2" long. The rear is more involved but much like the system I was thinking of. (click the 98 and older category)
Ford - Truck - Air Ride Suspension Systems - Kelderman Manufacturing

One questions for you guys. With that rear system with the swing arms being tied together side to side would allow for any side to articulation? I think you would still have normal movement with the shackle but I am having a hard time envisioning that situation.