Tom Endy (he is a member here) wrote some good instructions for rebuilding the A rear end. I am on dial up right now and it takes too long to open his page but a google seach will get you there;
https://www.google.com/search?q=Tom+...utf-8&oe=utf-8
As for your questions; the Model A rear axle gears DO NOT come off, they are part of the axle. The gears on the Model T axles do come off.
Why do you need to replace your axles?