Thinking further about this...
The slip rings are pretty deeply worn. And the brushes are awfully short.
I thought I'd like to do one more "test".
Brushes at napa are only 8 bucks. I called and ordered them. They'll be in, this afternoon.
I think I am going to put new brushes on it, and see what happens.
Kinda torn, this morning, though. I could just take the regulator off the other truck, and try the whole thing. But 1.) I don't know that the brushes are any better on that one, or not. and 2. If only new brushes on this one, "fixes" it, then I will have wasted a bunch of time. Instead of going to all that work, and then having to put the old regulator back on the old truck, in the end, anyway, I think I'll just wait on the new brushes, and swap only those, and see what happens. If that doesn't fix it, I'll know I need a new regulator.
I still have not found a part number for the slip rings for this. (Or a complete rebuild kit that looks like it is worth anything.) Regardless of whether the new brushes actually fixes the problem, or it takes the regulator as well... I'll still have to find the right slip rings and replace those.
I have found a new motorcraft regulator. I'll go ahead an order that, this afternoon or evening, depending on my findings.