The wheel bearing area on both axles are totally different.
A spare front axle would be useful only as a parts replacement source for another front axle...
...unless you are seriously into fabbing and your hobby money stash is big.

Most kits for replacing rear bearings contain the following:
Sealed bearing( races included)
spacer
press-on retainer
oil seal.
If you go to the auto parts store, they sell these item separately.
Optionally, you can purchase a SJ-410 backing plate that allows you to easily change the rear bearings without removing the brake assembly.
You can check all this out at this page at Roadless Gear:
http://www.roadlessgear.com/page/RGL/CTGY/DA