What year/trans? If I had to take a shot in the dark, I'd say maybe the shifter base gasket, or that silicone you used, or possibly even a hairline fracture.
It's hard to pinpoint in the pictures, degrease it and wipe it down then take it for a spin, it'll be easier to tell where it's coming from. If it's the tailshaft again, it'll spin trans fluid everywhere so you'll get a coat on everything; if it's just leaking from the casing somewhere else you'll see it pretty clearly.
2003 Mustang GT
K&N Cold Air/BBK 78mm Throttle intake/BBK long tubes/BBK cat less X-pipe/Flowmaster 40s