I had a shift shaft seal go bad many years ago. Took a while to isolate that one. Using a small mirror and flashlight look up between the inner case and trans to see a source. Just wondering, did you just change your trans fluid? If so did you put in 22-24 ozs.? If it's over full maybe it's just venting on you. Had a shop do that to me once.

 

I had my gasket set replaced 5x by the dealer(5th time a diff dealer)Found out the 1st dealer chewed up the shifter shaft and the new seals kept leaking!The last guy found it and replaced the shaft.It fixed the leak and only cost me $600.!!!!!!