Could this be your problem?
http://73-87.com/7387garage/drivetrain/yokeleak.htm
I had the same problem with my TH350C. I fixed it tempoaraily by cleaning the hole with brake cleaner, then using the tip of a screwdriver to dab some RTV on it. Later on, I bought the correct o-rings for it, and the problem, went away.
Slonaker