Apparently these seatbelts have a small gas or explosive charge that "pre-tensions" the belt in the event of a crash. When it activates it pulls the belt tight, holding you firmly in the seat. I called the local dealer and they wanted around $320 for the complete seatbelt/pretensioner assembly. I then searched online and found it for $205 at Varsity Ford Parts. The color is "stone".
I ended up ordering it online, and here's what they don't tell you: Because of the gas or explosive pre-tentioner, the part must be shipped UPS ground as "hazardous materials", which seems to mean it will take way longer than you expect to arrive, and it also costs an extra $50 or so in shipping. I don't recall the final total; it was under $320 (so still worth it I guess), but we ending up doing several family outings with everyone in the back seat (chauffeur style) during the two weeks it took to arrive. No joke, it really was two weeks. They had to ship it direct from Ford to the dealer who runs the website, who then had to turn around and ship it up to me.
In hindsight I did still save a few bucks, but next time I'll likely just go with the local dealer for parts.
Here's the new assembly, and the box it arrived in:
Replacing the assembly was quite easy. I used a 1/4" allen/hex wrench on the three large bolts. To remove the small cover on the shoulder D-ring you squeeze in the height adjuster buttons and pull the bottom of the cover up.
