2004 Tahoe LT died while driving

I would immediately pull that vehicle from that shop and find another that has a reputation. Check google reviews, friends opinions, etc for a reputable shop.

While not impossible, the odds that your PCM is causing this problem are close to ZERO.

PCM --Powertrain Control Module (AKA engine computer)
BCM --Body Control Module (controls everything from interior lights to in the vehicle amenities and your security system inteface.
DTC --Diagnostic Trouble Code -- (AKA your check engine light is on and means a DTC is stored in the PCM)

2 things. Years ago when the check engine light came on we went right to the shop. They checked and there were no codes. I saw the code reader.
They said the check engine light can malfunction.

i was an Air Traffic Controller in the Marine Corps, and had a few emergencies where a chip warning light came on in a jet fighter, and it always turned out to be a malfunctioning warning light, so it seemed right that the check engine light can malfunction. That's why we didn't worry about it.
We later bought a code reader, and it also showed no codes.

I'll check tomorrow if there are any codes now, but if there were, I think it would have helped them diagnose.

If there's no progress tomorrow I'll have it towed to another shop, but I don't know how good they are. Many years ago I had a different car in that shop and was unimpressed, but one neighbor swears by them.

This shop has always been good, honest, and fast, until now., so I've been trusting them....

The most likely cause of a fault like this is a bad ground - have them check the main engine block ground if they haven't already, it's behind the intake manifold close to the firewall

Well I talked to them today, they want to take it to the Chevy dealer.
They said the dealer has more specialized diagnostic tools than they do. Apparently there are problems in the computer itself that they can't fix. The codes aren't showing because the computers are damaged, and even if they bought new computers they would still have to get programmed by the dealer.

They have fixed problems they have found, but still stuff beyond their capacity. They think a short caused damage/more shorts to multiple systems, all at once.

We have had a good relationship with this place for years, and I have left it for so long because I know they are honest and aboveboard.

So, the saga continues. But I trust them more than a dealer.....