There are a huge number of axles that will fit. The things to look for are the width, the bolt pattern, and the gear ratio inside. All of these variables can be changed to a degree, but it's extra work. No other axle will be a direct bolt in for that CJ7 unless it's from another CJ.
Also, I think you mean AMC 20 rear axle. Tha Dana 20 is a transfer case. I'd hate to see you buy parts for a transfer case when you need parts for an axle.