The old differential fill plug was a little leaky. I replaced it with an ordinary, galvanized 3/4" NPT pipe plug from the hardware store ($0.98). The plug is taper-threaded, but for good measure I put some Teflon tape on the threads. No more drips!
underbody needs a good cleaning
While I was under the car, I drained the differential and transmission, and filled them with fresh 80W-90 gear oil. (Everyone loves Redline MTL, but the local parts store was out and I'm cheap, so I used a synthetic blend from Valvoline.) FYI, there's no drain plug on the diff, so the fluid had to be siphoned it out.