Do any of you know the part number for the 2005 fog lights? I need to replace one of mine that cracked. Also, do you know of an online store where they can be purchased?

www.xterraparts.com just look for the fogs, they sell the assembly.

If they came stock (which I assume they did on your OR) and they cracked without abuse or without getting hit by something, they should be replaced under warranty. Just cracking for no good reason (like just a temp change) is not acceptable...