Good question. I too have been pondering on a good section just for axle's, as the Transmission / T-case section doesn't really cover these. We don't really have a general cross use section, dedicated to just axles. Just Off road project builds.
A subforum in the Street trucks section would help. One could show axle builds for street use, showing traction aides, gearing, shafts, brakes, bars, etc.... The Stang race crowd are big into these 9" axles, but I haven't seen too much use for the Ex, other than Off road use, for full width applications.
For now, one of these should work, depending on your preference.
For Off road builds, in the Off road projects drivelines.
For racing/street use, seems the only fitting place would be in the Need for speed.
For a specific generation use, the modified sub forum.