99 a8 rear diff
 

Looking for a little advise. I'm running a 99 a8 109000 miles. She's been really good to me no major issues. Do most maint and small fixes myself. Happy to say no tranny probs. Did the rear brakes this weekend, when I was under there just did a quick inspect and noticed the rear transfer case looked a little wet. looks like some seapage. Thinking the lower grain/plug or a seal. is that common to see? Is there anything I can do myself to troubleshoot or should I take it for service? I wanna do as much digging as I can before I bring it someone.