Sometimes, I'm told, even as the oil level in the reservoir goes down slowly, it's not completely awful ($$$) news: there's two hydraulic lines from the reservoir (one in / one out) to the transfer case and it could be a small external leak in one of those or their union at the TC, fairly simple repair.

Also, even if the leak is internal, it's not necessarily death for the system: change the ATF fluid part in the late fall which will purge any hyd. oil contaminating it, use it in the snowy months, and then turn off the system when the winter weather is past (flip the test lever up). Unless the system's internal seals are totally shot, the inter-chamber leakage can be so slight that the thing works well...remember the TC is engaged only about 1% (or less) of the time, even in winter.

I've done this for 5 years and have no problems with system, and now that the roads are crappy, I'm pretty happy to have it in good order.