If you do not charge 12.3+ vdc at idle with lights on (without increasing the rpm of the motor/alt), I am still of the rather firm opinion that you will find either a bad ground or a broken cable (inside the insulation), my bet is the large cable leading to the back of the alt has a break inside the insulation. But the ground leads are a close second.
If this truck is an "LPS" rated version, then you may have bad (usually the positive side contacts) contacts inside the mechanical/electrical isolation shutoff switch assembly on the positive side, and it is able to carry a light load, but dropping out load when loaded up. At idle, when just started, without increasing the rpm of the motor/alt, use a battery load tester and see if the volts stay above 13 v when loaded for a few seconds.
This may also be the diode set inside the Alt having gone bad (especially if it is a sealed alt that has been rebuilt at a local re-builders). I have recently stopped trying to have the sealed Alt rebuilt locally, and for now I am only replacing the sealed (LPS) version Alt with factory new Alts, (not even Promatch rebuilds) as the diode pack that "looks" to fit has had repeated failures (for me) to the point that the local re-builders have given me my money back and declared they can not rebuild that Alt. (yet). If you want my guaranteed fix, you would replace the eye at the end of the large cable to the back of the Alt, replace the LPS shutoff switch assembly [if so equipped], replace the alt with a new (not rebuilt) unit and replace the battery with a new battery, since it has now been run dead a number of times. If you still have this intermittent failure, I would change the 2 wire plug connecttor in the back of the Alt.
You can beat me up in public (after waiting in that rather long line of people who want to beat on me first ;-) ) if that does not fix it for you.
If you still want to change the vcm or ecm, knock your self out, (don't let -me- stop you) but if you do, and it it fixes or changes anything, please come back and let us know. (if it _fixes_ this problem for more than 2 weeks, I promise to be amazed, and will remember what it was that you did that fixed it)
I would have also mentioned a waterpump going bad or loose belt, but since you offered up the 14.6vdc charge rate, I figured your belt was tight