tricking doesn't always work, most gm's when it isn't truly sensing the load it will not put out fully. IT must be limiting field current more or something.
in my 3.8l car when its charging 14.4 my alt does over 300a, but when it backs down, i turn my headlights on to trick it, then voltage goes up to 14.0v the alt will only do 150-200a at best until it reacts to the voltage drop and starts charging in the 15's. then its right back up around 300a output again. I've seen this in trailblazers as well. haven't tested any trucks, however.
send your alt in and have it converted to a pcm bypass setup and get rid of that pcm crap.