The truck will start up and drive fine, but the headlights flicker and are noticably dim. However, when I take my foot off the gas and press the brakes, the radio cuts off and the headlights get brighter, both for about half a second. If I drive it like that for longer than 10-20 minutes, then check engine light will come on.

I've replaced the charred positive battery cable lug, replaced all three fusible links (they weren't broken, but the 20v looked a bit corroded), cleaned the crud off of the battery terminals, and disconnected the negative battery cable at the engine to clear the crud off of it at that end.

The only thing that makes sense to me is that it could be the voltage regulator. After taking a look at the Hayne's manual, it looks like there are two kinds: one that attaches to the back of the alternator, and another that's on the inside of the alternator, and it doesn't specify which is for which years. I think that mine (for a 1995) is on the inside, which means that I'd be better off just replacing the whole alternator. Am I on the right track here?