Likely places for leaks are;
Input seal, which is in the oil pump, or running gear front plate gasket.
Bottom plate gasket (this plate supports the bands so requires complete strip of box)
Side cover on selector side.
Drain plug.
Top cover gasket.
Rear face gasket where box mates to transfer box.
As the oil is being flung up, I would think it is from within the bell housing, so removal of tinwork in that area should reveal the source.