Changing the locks is a simple way to go, but now you will have another key to keep up with... Bummer
I agree with Ashman, probably superglue or other Cyanoacrylate product. My college roommate once had this done to his T-Bird. A local locksmith was able to clean up the locks in about 2 hours. This included removal, cleaning, lubrication, and re-installing. The cost was cheaper than replacing one of the locks! Check around your area for a reputable locksmith.