The axle appears to be a Waggy D44. In '79 or '80, waggys switched from a passenger to driver side pumpkin. Any axle after that makes a good candidate, but avoid the vacuum disconnect ones (they have a box looking thing mounted on the pumpkin itself). I got mine complete with steering, manual hubs, and brakes at a yard here in Phoenix for $235 (pulled it myself). Prices vary thoughout the country, but seem to be the highest in the southwest. Waggy axles are 60.5" wide WMS to WMS which is about 1.5" narrower than the rear end, however that's negligable.

This kit looks awesome. Too bad I already SAS'd my Fronty, but I do have an Xterra....HMMMMM.