I've got a seriously beefed 700R4 awaiting it's finaly resting place in between the frame rails of my M1008. Even with 39 inch tires, it still needs OD bad! I'm averaging 14 MPG right now as best I can tell with the odometer not reading correctly, which aint bad for some big tires like that and they're pretty heavy. A 700R4 swap aint too hard, you just have to search for certain diesel specfic parts like the torque converter, tall transfer case adapter, and the TV cable bracket for the 6.2/700R4. You'll also need to swap input shafts on the transfer case or just swap in another one.
Try an intake mod and some decent exhaust and see what happens then go from there.