The winning combination pictured below: 14" 1/2" drive breaker --> 1/2" to 3/8" drive adapter --> 8mm 3/8" drive hex key
-----I had the Craftsman breaker, purchased with points before all of the local branches when under
-----I picked up the adapter and hex socket from HF
I tried to open-palm the breaker bar as much as possible, but still managed to slightly bust my pinky knuckle as the plug let go with loud POP!!!, ripping way a 1/4" of skin in the process...
The next hardest part was getting the diff full and THEN lifting the car to re-install the overflow-check plug. I had to lift the rear up and shimmy under, since the overflow/check plug is on the rear driver-side of the front diff. I knew I was on the right track when I began to lift and the slight dribble through the port, indicating that the diff was full, completely stopped.
MAN DO I MISS HAVING ACCESS TO A LIFT!!! The Forester front diff was MUCH easier to do, because the fill port is the vent port, which requires pulling a vacuum-style hose which is connected via o-ring seal only. There's also so much clearance that I didn't have to lift to vehicle to shimmy under and re-install its overflow/check port.
For reference, the plug is just above the driveshaft, which is covered with a green rag in the first pic, since I was spraying Deep Creep and PB Blaster over the preceding 48 hrs.
Here you can see the cat and exhaust right beneath, leaving very little room to barely get my hand angled in past
Here you can see the plug (without the hex installed), above and to the left of the driveshalf