This evening I got the rear differential all closed up and refilled with gear oil. Last time I did this I added the oil through the vent hose at the top of the differential housing, since there isn't a fill plug on the differential or on the cover. But I could not really tell how much was fluid was in the housing, so I ended up over filling it.
Per instructions I found somewhere on the 'net, I looked all over the differential this afternoon and I FINALLY found it. Yep, there is a fill plug on the differential. Right where it is supposed to be, on the top corner of the passenger's side facing the engine in a forged, recessed area.
So I figured I might as well use it to fill the differential back up. And even though the bed is installed it is a lot easier this was than the last time I did it. Best of all, the gasket was the only part I needed to buy. Gotta love a $4 repair.
Once I got everything buttoned up, I went for a short cruise to the gas station to buy some fuel for the lawn mower. While I was driving I heard the nasty grating noise again. I still can't tell if it is engine, clutch or driveshaft noise, though.