Well, I got my truck fixed today. Turns out it was the ignition module, which sits underneath the rotor on the distributor. It took $80 to get the part, and about an hour to change it. Now, all that's left is new exhaust.

I did notice, in the process of looking for the problem, that there's a lot of wires wearing through from rubbing against something. Also, my valve cover gasket is leaking a lot worse than I expected.