Nate,
If you are lucky you will only need a new brush/diode set... so as soon as you install the new one in all liklihood you will own it for most parts houses do not accept returns on such unless they are obviously defective. So as soon as you put the new one in, whip out your volt meter again and check the voltage with the engine running and it should be 13.5+... as per "diesel don"'s POST earlier.

If you do not have 13.5+ voltage out of the alternator then you probably have a defective armature or field windings and that most likely means a new/rebuilt alternator.

Good luck and keep up up on what you do and I'm sure you will get more advice.
Sam