The 8 1/4 thread

In 2008 I bought a '74 Duster with a 2.45 geared 8 1/4" axle. Later I pulled a 3.55 SG diff from a Dakota 4wd. Even with limited skills, I switched the diff and gears into the axle and it all fit and worked. THIS tells me that the axle housings and center section of those two were at least compatible. It also leads me to understand that it is the differential itself that is different.