His truck may not be the SD, it may the the 7700lb GVWR version, I believe it was available with the 4.6L V8.

As to your problem, I have no idea. It's obviously wiring, take a big swig of patience and start at the beginning, eliminate everything till you find your problem.

A dealer/mechanic will do it, but for the same reasons you don't want to do it, it usually gets really expensive.

Good luck,

Waxy