Whoops, forgot to update this thread.
I tried turning the starter to another position but it didn't do a thing so I pulled the whole starter and successfully tried it on the floor. I also visually inspected the field-to-brush wires but didn't see any evidence of a short. The starter worked fine on the floor, so I figured I'd toss it back in the truck. Since floor-starting it put a little bit of slag on the copper lug on the starter housing, I cleaned it up with sandpaper instead of just scotchbrite, and I think that may have been just what it needed. The starter worked just fine when it went back in the truck and has worked fine for the past 15-ish start attempts now.
