I've always recommended replacing the entire axle. Once your boot is cracked/torn, there's no telling what kinds of debris has found its way into your grease and joint.

I picked up remanufactured axles from Auto Zone for about $85 or so. They come with limited lifetime warrenty and I've already had to replace one under that warrenty because the boot split prematurely. The warrenty was honored and I've had no problems with the new axles since then.

Unless you're putting out over 200 or so WHP, you can stick with either OEM or even rebuilt OEM axles.