Fluids need to be changed when they need to be changed, what ever miles you put on your ride offroad may put a little more stress on the parts that have changable fluid in them, so if you do a lot of off roading you can maybe change them every 2500miles instead of every 3000 or however often you do them. But engine, tranny, t-case don't need to be changed every outing, say again do not need to be changed. Now as for the difs, if you are in a lot of water, n mud you may want to change the dif fluids more often on the front d30 you have. Unless you have something extra like the Superior Axle Seals to help keep crap out of there, they will take water n mud in thru the axle tubes whenever you cross the wet stuff. Result is milky, ugly rather useless dif fluid that you'll want to change more often.
other than that I would say your good to go, I don't change my fluids any more often than I did when I didn't offroad, and my ride runs better than when I bought.
enjoy your ride, on the road and off.