My guess is that if you attempt to switch over to air brakes, you will be putting A LOT of time, money, and reverse engineering into a vehicle that will get you laughed out of ANY .gov inspection station. Chances are they will demand the truck be put on a "low-boy" and and never to be used on a public road again.
Besides that, hydraulic over hydraulic brake systems work GREAT and are much easier to maintain (especially dual systems). I didn't like the idea of both my steering and brake assist being lost if the power steering pump failed so I split the system by adding another pump/ pulley/ belt for the brakes. "baby food" compared to re-plumbing a truck/ motor for air!
Your best bet is to stick with the correct "sized" driveline components when reconfiguring your truck to 4X4 ($x$)! But remember that you will battling some really challenging alignment issues. Most professional conversion houses "Z" the frame (cutting the front of the frame off the back and stacking it on top of the rear frame), thus buying the space for the transfer case and front axle under the engine.
Honestly, unless you have the facilities (like I-beam overhead gantry), tooling (like a way to fixture the frame on a flat/ level/ square surface), "top-shelf" 3D modelling software, AND WAD OF MONEY that you will most likely be wasting... I'd just put a locker in the rear axle, super-single wheels/ tires, a big stack of Bengy's in the "tractor fund" and go camping!
