Just uploaded a bunch of MSD wiring schematics and information. You can check it out here.
I think I found your noise. This is the ring gear out of the front PTU unit (transfer case) in a 2001 Chrysler Town and Country AWD van we had come into the shop. This is the result of driving with an under-inflated tire on an AWD vehicle. Proper tire inflation, and running all four of the same size/type tires is incredibly important and will help prevent damage like this. This particular repair was upwards of $2,000 and could have been quite easily prevented.