[LIforrelaxin]

Been here done this, had close to the same problem, here is how the scenario played out....

1. One fall the plug didn't come out cooperatively for my mechanic.
2. We discussed the issue and several remedies.
3. My mechanic spent the winter asking around, because neither of us where excited about the cost of VP lower gear case housing....
4. Heli-coils where ruled out, the fail no one would recommend that fix.
5. In the spring we decided to roll the dice, re-tapped the threads, and filled her up and put the plug in.... it sat in the yard, and a little drip was noted.
6. Pulled the plug, and put some "goop", this is all my mechnic would refer to it as, and a new o-ring.... filled her with oil...
7. the ice was out and there where no more signs of oil leaking... in she went.
8. Ran like that for 5 or 6 years no problems... Still could get a definition of the "goop"......
9. then one day the plug came out, and didn't look so good, threads in the case seemed fine.... so a new plug was put in, sealed up nice and tight, a pressure test was done, and passed, with no "goop", ran all of last year just fine.....

So my thought would be to chase the threads, get a new plug, and if you have the ability pressure test it do it... if not fill her up with lube, and let her sit for a few days, and see what you see.