If it's leaking at the tailshaft, it's definitely transmission fluid.
Most times that gasket goes bad, it's due to the tailshaft bushing going bad, in turn allowing the seal to get more abuse and go bad as well.
I'm getting ready to do one on a TH350 and I'll do a write up when I get to it.
This is what the kit looks like for the TH350, usually costs around $35 USD for the kit around here.