JD,
This is a stretch, but if it's just the fuel pump, you could have broken insulation out of the tank along the frame, and some kind of conductive buildup between the pump wiring and ground? Like undercoating or brake fluid?
That would be so unusual and rare that I doubt that's the trouble.
Something else HAS to be connected to this line other than the fuel pump. did you try removing the relay? Water and othe conductive materials have been known to get into relays, but they usually dont function. It could be stuck in the "on" position?!!
It might help to spend $25.00 and get the schematics for your truck at
www.alldatadiy.com and look at the circuits.
One sure fix: use a switch and cut off the fuel pump circuit when the truck is sitting. We used to do that for a cheap security system anyway...
I don't know of any of these systems ever using a diode or capacitor in the fuel pump line. If there is one, it would be IN the fuel pump assembly.
happy troubleshooting!
Slantsixness