Ok, I was able to work on it tonight and got it working...for a bit, then it failed on me again.
I first did as many of the tests as I could on the starter in accordance with the shop manual. Everything I could check tested good.
I cleaned all of the mounting surfaces up in case there was a grounding issue, re-seated the brushes to the commutator with some sandpaper, and cleaned off the battery cables and mounting surfaces on the battery and starter for the 100th time just to be safe.
One of the brushes was pushed down a bit (forward on the starter with said starter installed) so I pushed it back to its correct place. It had a sizeable ridge built up where 1/8 of it was riding off of the side of the commutator, so I filed it down before reseating all of the brushes with 80-grit then 400-grit sandpaper.
One thing I noticed is that the wires that connect the field to the brushes float pretty close to the starter case and they'll cause a short. I was careful to install them so they didn't contact the starter case and check it with a multimeter after the starter was completely rebuilt to ensure there wasn't a short between those wires and the case.
I "bench-tested" (On the floor for me) the starter using a spare battery and leads per @Hotrod Lincoln's suggestion - thanks - and it worked.
So I reinstalled it in the truck and tried it, and it worked at first! I got about 30-seconds (non-consecutively) of starter action but on the last start attempt when I let my foot off of the pedal the starter coasted to a stop and has refused to make any movement since then. Bah humbug!
I've heard that armatures can have a "dead spot" of sorts and won't do anything if the starter is turned a certain way. That is my first suspicion for my starter. Tomorrow I'm going to pull the starter just enough to spin it a bit and see if that'll let the starter do its job again. This seems like the most likely issue to me since the starter only stopped working after I let off of the pedal and it came to a stop naturally. I don't have access to a growler, so unfortunately I can't test the armature any further.
If that doesn't work I'll probably check on those field-to-brush wires.
