There are aftermarket kits available. I have never used them in a 4x4 application. I am sure some others here have more info with regards to durability and reliabilty in an offroad application.
Here is a result from a Google search:
http://www.accessconnect.com/air_lift.htm
BTW, you can just go with a full lift. There is lots of info on this site. Just use the search area.
Black Lingenfelter Lux