I was able to pull the hub off the rear driver side and this is my current axle. Is this mostly surface rust that can be removed? What's the general requirements for a rebuild?

 

just get some sand paper, or a mild grinder, paint it if you like and it'll be good. just make sure you cover the threads on the bolt if you do paint (which there really is no point for on the hub)
if you use the sand paper or grinder and see shiny silver, your all good.

 

Wire brush and por15 it but for me i never bothered to paint mine no one will ever see it.The housing and 3rd member i did paint tho.

 

awesome. I'm glad I don't need to buy new ones. What would be the reasons for new axles though?

 

broken, twisted, bent, damaged splines, cracked, or the important machined surfaces are damaged (mostly where the bearing and seal goes).

rust, not so much. the exposed bits always rust. they lead a hard life.