After you can no longer convince yourself that the transfer case is supposed to leak like that, it will be time for new shaft seals.  There is no need to pull the transfer case but it will be a lot easier if your remove the skidplate.  Click here to read how to pull the parking brake drum to access the rear seal.  For a complete rebuild, you'll need Rick Stivers' rebuild guide.

The protruding shaft will get in the way when replacing the seal.  You cannot grip the inside of the seal with a slide hammer like inside the axlehousing.  The old seal can be pulled with the shaft in place by the use of a hook type seal puller.  This picture shows a rear bearing cap for a transfer case without a parking brake drum, but the procedure is the same for either type.  Don't forget to drain the oil before removing the seal:

If the rear seal refuses to budge, Jeff Sovich told me what worked for him.  By unscrewing the speedometer drive gear, he was able to drive out the seal from the inside with a hammer and punch.

The seal's mating surface on the driveshaft yoke must be smooth and clean.  Click here to see a similar yoke for the rear axle, including info on a Speedi-Sleeve repair.  The original seal can best be described as a mere amusement.  Rick Stivers, the author of the T-90 and Dana 18 Rebuild Guides, recommends a double-lipped National Federal Mogul oil seal Part # 473229, which fits perfectly in place of the original placebo seal.  Check your Yellow Pages under "Bearings" for a local source.

 

Now you may ask yourself how can a guy install the new seal with that shaft in the way?  Why, you need a Vernco Seal-O-Matic 2000:

Here it is in use.  Sorry, but these heirloom quality beauties are out of production.  A piece of PVC pipe would work well, too:

Now go forth and leak no more.  When reinstalling the yokes, don't forget to add a smidgen of non-hardening sealer to keep oil from migrating out through the splines.