You'll need to replace the pinion gear AS A MINIMUM, and if any other differential parts are damaged, they too will need to be replaced. Your best option would be to find a replacement differential and drop it into the vehicle. You can usually find one at a local wrecking yard for quite a bit less than new. Optionally, take it to a shop that specializes in setting up differentials. If the lash is not set up properly, your differential will make a terrible noise and will fail prematurely. If you don't have experience/training, don't try to do it yourself.