New Guy with t-case Qs

[DruBlazer]

Hey folks. This is my first post.
Intoductions first: My name is Drew and I have recently bought a 71 4x4 Blazer after spending years looking and wanting. It currently has a little lift with some engine mods, but besides that, pretty stock. Which is how I'd like to keep it. The guy I bought it from didn't know much about it, or any mechanical thing, at all. And I can tell. He did some things that I've spent time undoing.
That leads me to my question. As I'm crawling through my rig I noticed that the output shaft on the t-case is leaking. So my thought was that until I can fix the leak, the least I could do is keep it full of gear oil. When I pulled the fill plug, not the drain plug, ATF came pooring out. Could there be a problem with both the output seal of the TH350 and the input seal of the 205 that would cause the ATF to get into the t-case? Has anyone encountered this problem? Ideas?

Thanks