You've probably checked theses basics Allready but.. Any sparkled in he tranny fluid on the dipstick. You could pull the cover off the torque converter and check for end play and broken flex plate (not likely but quick) metal in cover. And drop pan and check for small parts and shavings/sparkles here. Are you FWD? Might try shifting 2-4 and try to determine of it is xmsn or xfer. I suspect leak you mentioned earlier is the best clue though.