If it's not leaking somewhere forward on the motor and running backward (which is entirely possible) it's probably that freeze plug mentioned above. Nothing else back there to leak unless the head gasket is super blown, and you'd probably know that already. Like was said above, pressure test it. Go to the parts store, rent a kit and pick up a bottle of degreaser. Stop at the wand wash on the way home and clean off the motor real good. Drive enough to dry everything on the way home. Let it cool then test it.
And fwiw, I had a slight drip there after my motor rebuild and was pretty distraught since the only fix involved separating the motor/tranny. I tried GM coolant sealing tabs and they worked great. It's not the same as some of the things they sell at the parts store that can potentially clog up your system. I'm sure you'll still get some people who will say it'll clog everything up (without ever having used either but hey, internet knowledge). GM ran those from the factory in every car of certain makes for years, so that should tell you something.
