Well, shit. I broke one of the diff cover bolts inside the axle housing.
Good news, it's not leaking. Better news, I got the oil changed in the differential for the first time since I bought it and then some, probably five years or more. The drain/fill plugs looks like it will accept a 1/2-inch socket, but it's tapered on the inside. I used a 7/16 square key from the hardware store that fit just right.
I found a seeping rubber brake line going to the proportioning valve that was leaking. I thought my brakes felt a little mushy. If the rain holds off, I should have this mess finished by Sunday.