This sounds like a short associated with engine position to me.
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Any word on truck?
have you ruled out a bad diode in the alternator? i would also like to see 2 fresh batteries on it to see if 2 batteries inline would dampen the electrical spike so to speak.
is this an early 3rd gen with apps? maybe timbo can help you .
did you wiggle the ignition wires?
there is no way to have the dealer "test" an ECM.