2003 DCSB Door Lock Noise/Won't Lock

The title is pretty self-explanatory, but for detail:

Sometimes I lock the doors or when they go to lock automatically (shifting into drive, namely), the door lock on the front driver's side door will make this horrible squealing/grinding sound before locking. I'd say about one time out of 50, it will fail to lock properly. It's been getting progressively worse over the last few weeks and I plan to pull the door apart over the weekend. I was just wondering if anyone here had experienced this kind of issue before and if so, what the cause and fix was.

Thanks!