I recently did the drum brakes on my 1st gen and I read the forums and everybody says the axle nut size is 2 11/16, and that is also what comes up in the parts stores. When I took the nut off, I noticed the socket was a little large, it still worked since I was in a pinch, but was there a year when things were a little different? It's a 90 D250, stock as far as I know, there are no markings on the diff.

 

Axle nut size is 2 9/16, rounded corner.