For what it's worth my transmission also leaks between the transmission and transfer case, but never did before I had it rebuilt and I never had a gasket or gasket material between them in the fifteen years I owned the truck but then again I got a dumb th 400 with a 27 spline output shaft. Go figure th 400 is supposed to be 32 spline, though they did make a th 375 but was a 2 wheel drive car trans that looked like a 400. I'd say rear output seal or something is cracked or broken.