hello, I know this is a really old thread so I am doubtful I will get a reply.
But you stated the trans and T-case share fluid but I think this is not correct. The transfer case in any transmission configuration AT or MT is lubricated with hypoid gear oil.
The auto transmission is full of dexron and MT is also hypoid gear oil like t-case and differentials.
I came across your reply because I am having dexron somehow leaking or being forced, from the AT into the void between the T-case and transmission where dexron should not be.
Obvious culprit one would think is a bad rear main seal but the main seal looks good.