I have one that was doing the same thing. Fortunately I caught it early. The vent tube on the transmission was stopped up causing the transmission to build up pressure forcing fluid past the seal into the transfer case. I ran a new vent line on the transmission up to the fender well beside the blower motor and monitored it since. While I was at it, I ran another vent line for the transfer case.
If I remember correctly, the vent for the transfer case and the vacuum release from the three port valve were connected so I separated them. I stuck a cheap plastic fuel filter on the end to keep dirt out. This all happened about 7 years and 70k miles ago. Still monitoring fluid levels. Transmission and transfer case still original at 203k.
This post has some pictures of that vent tube. When I replace the transmission on the Blazer I did the vent lines the same way as the crewcab.viewtopic.php?f=14&t=8777&p=96041&hilit=vent#p96041