Let's get onefast up to speed. There's 2
engine oil drain plugs. Looks like you found the crankcase one already. The other is on the bottom of the oil tank... pretty far back on the bike, and on the right side.
You've already identified the transfer case, and the info here should get you right on track with that, and changing its gear oil.
Follow the oil change instructions carefully. Drain from 2 plugs. Put only 2 qts in the oil tank filler and start and run the engine, with a few throttle blips, for 30 seconds or so, and then shut it down. You should then be able to add the second 2 qts in the oil tank.
By NO means start and try to run the engine with the oil tank dip stick out. If you do, no one will need to tell you why!
Do not ask me how I know this.......