HD4 stopped moving

I was operating my well used AC HD4 a few months ago and it began to move slower and slower and then finally stopped.  The engine was running, all hydraulic controls were fine, it just stopped moving.  I messed with shifting the gearbox and pulled the forward reverse lever back and forth and nothing solved the problem.  So I shut it off and climbed down to see what I could.  First thing I noticed was that clean oil was draining from the skid pan onto the ground.  And then I saw a round hole in the transmission casing which was not plugged but was wide open.  I found a wooden plug in the dirt which I assume was pounded into the hole at the factory.  I think the plug blew out!  So I started the engine and saw that oil was flinging out the open hole at a pretty good rate.  What do you suppose let loose inside the case?  It seems that the transmission case is getting pressurized somehow?