Rear axle seal after it got pushed out:
This little bugger - and my own stupidity - was likely the cause of the problem:
It's the vent for the rear axle housing. It's up under the seat near the pressure-relief valve for the hydraulic system/3PH.
You can see the drain hole in the end:
A while back I stripped off a bunch of the plastic panels and the seat to thoroughly degrease the tractor. I found that the little rubber vent tube had deteriorated and had partially crumbled apart ... to the point that there was a vertical path into the rear axle for water to get in.
So I figured I'll just temporarily cap it with a vacuum cap - so I can wash down the tractor - until I can get the part to replace it.
Well, it took them a little longer to get the part than expected - and then once I got it, I didn't replace it immediately. Very bad move.
I think what happened was that it caused the axle to pressurize, which caused the rear axle seal to get pushed out. I'm actually surprised that it didn't just push the vacuum cap off.
And, of course, it caused any moisture in the air inside the axle to be retained ... which ain't good.
If you have a tractor that is older (ours is 10+ years) I'd suggest looking to see if you have one these somewhere up under the seat and then inspect it's condition.