Thank you, and you are welcome. I used to check voltage drop as a mechanic in the Army, and I'm pretty sure my father in law knows how as well, if not I'll give him a hand.

Not much more that I can think of right now without sticking my paws on the truck.

My next step would be to follow "hanky's" resistant checks. A cable with high resistance can cause your issue. And the hotter the cable gets, the higher the resistance (Thermal resistance). You could also run a ground cable directly from the battery to one of the starter bolts to see if the issue goes away. If it does, then you have a bad ground cable/connection. Being almost 20 years old, a new set of + & - battery cables would be a nice New Years present for the truck.

Well put, "Use Common Sense" , you do have a way with words. (LOL)

I get a little hesitant when talking about resistance because there is always going to be someone that is going to try to measure the resistance of a battery cable with an ohmmeter and you know how that goes.