Gee, could the 1200 lbs difference in weight along with less aerodymaics have an effect on the fuel mileage?
Even with a part time four wheel drive system, the front axle is turning the diff, causing friction, losing fuel mileage. I really don't believe you will save that much for the switch due to the above mentioned factors.
I would guess the only sure way to improve mileage would be to make it rear-wheel drive only.
(Why would anyone want a rear wheel drive only Hummer????)