there's a gasket on the transfer case side and an o-ring on the transmission side. in addition, there are 4 bolts attaching it to the rear of the transmission. each of those bolts has an o-ring on it as well.
clean it off and see if you can see where the leak is coming from (ie: front side or rear). not too hard to replace any of the stuff in there. i elected to get all new gaskets and the 4 bolts that hold it to the back of the trans since i didn't like the way the stock bolts were corroded.
gasket - 15642511, lists for $9.98 at dealer
bolts - 11517778, lists for $6.95 each and are special order.