Dealer has had it two days -- he says "no codes" ... so, I talked to the mechanic and asked him what he'd do if it was his truck and I took his computer away.
He replied with what I believe are the right answers, and also did it when he found no codes -- check pressure and lines, and a few other things ... he's keeping it over night and will try again in the AM to get it to act out.
His thoughts are: Bad gas on a fill up ... which carried over into another tank - so, now with new gas, maybe OK. I'm having a tendency to believe that for now. Very possible, altho' I usually fill up and brand names on the well traveled highways ... but any station can get a bad batch, I suppose.
Far as asking only here -- nope, ask other GM sites as well. More just to discuss it than to truly find "an answer" to it. Just see if others have had anything like it, and to share the dilemma and share the results - so others with GM trucks know what happened ... Not truly looking for someone to tell me "OH, just do this." That's not very realistic. But we always appreciate the advice and the discussion ... and always (usually) learn something from it.