There is a seal in the rear of the main transmission housing. Your's is probably leaking. The transmission oil is more likely to be accumulating in the adapter housing, and not entering the transfer case itself. That's not to say that it cannot enter the transfer case, but it probably isn't there just yet. If the transfer case oil is more red than green, it probably has some trans oil in it.
To repair the leaking seal, the drive shafts and transfer case need to be removed. When you remove the transfer case, expect to find about 3 quarts of ATF in the adapter housing. The transmission rear housing extension/adapter must be removed. With those parts out of the way you can see the seal:
Unfortunately, with the AWD and 4WD variants, the seal is part of the low-reverse clutch hub seal (at least it was on mine), and to replace it would require removal and disassembly of the transmission (been there). Before you go that far, it may be worth changing trans oil and adding seal conditioner, or better yet, switching to synthetic trans oil to help soften the seals again. Of course, you'll want to check the fluid level regularly, like all other fluid levels.