Correct me if I'm wrong but can't the whole race be tackled in 2wd? Baja bugs to trophy trucks and everything in between does. I've never been out there so I don't know the terrain well. I still see a benefit in 4wd in case you stop in a silt bed you have the 4wd to pull you out where I would assume momentum is key in the big silt beds.
Anyways you don't even need to pull the diff. In a racing situation where time is key pull both halfshafts and driveshaft and put it in 4hi locked with the vehicle off (computer wont interfere when it's off. If its on the computer won't engage into 4 hi locked and in 4hi unlocked it will send the power to the front end.) also the way the H2 diff is setup the stub shafts will contain all fluid and broken parts so you don't have to pull the whole thing.
I would be more concerned on steering. It's a heavy truck and pounding down on that front end may put the steering to its breaking point. Is stock steering required? Can u just put beefier tie rods and idler arms on?
I also hear H2 steering systems get hot so a cooler may be necessary.
Lastly, good luck! I'm glad to see HUMMER being used in racing situations