Are you seeing an oil leak or are you actually seeing the seal come out of the bore?Did you run a vapor test on the PTO side? I know a lot of guys around here are experiencing something similar with the pin side of the balancer weights. If you are getting positive pressure at your sample valve, I would say you need to look into your carb. Otherwise, someone just probably installed the ring gear improperly. Sometimes it takes a while for this to cause a proble. Trust me, I have seen it myself!