Son of a gun. Isn't it amazing how nature will figure out ways to surprise us, if we haven't thought of it all? That is most interesting.

I'm planning to instal electric fan/s to replace the belt driven/clutch type in my '92 and the controller I've seen most commonly is the type with a probe that goes inbetween the rad fins - I had considered putting dielectric grease or maybe even heat sink grease (not sure about the latter and electrolysis) to protect both, but the hint in the article about looking for leaks to earth is a good one...I'll try that myself.

Further to their suggestion about lining fan boxes with foil, there's a conductive metallic paint you can spray onto plastics to improve their shielding, it's usually available where you buy electronic construction/manufacturing and hobby supplies...aluminum flyscreen might be another sturdier option.

 

I would attach a ground wire to the radiator and chassis.
I know you have a new truck now but this may help someone in the future.
Galvanic corrosion is a real problem.

 

Bingo!

Anytime you have to replace either the heater core or the radiator, make sure the replacement has a ground cable soldered to it, with the other end going to a clean chassis ground.

 

Has it ever run hot? Have you replaced the thermostat? Random train of thought here, but if your T-stat is stuck for whatever reason you could be stressing the radiator at the seams and they eventually fail at a common weak spot.