Hey, apologies for the extremely late update.... and resurrecting this thread but I have recently returned to this forum and wanted to close this thread out properly.
Anyway, I ended up replacing the fuel sending unit (again) at the GMC dealership... around 20 months ago, along with a ton of other upgrades I added myself which I don't really want to list in this thread. Pics and info are coming in the appropriate thread.
Long story short, I did the repair at a quality, (and equally expensive) repair shop and it seemed to make a huge difference. I have put roughly 50000KM on the Shimmy since the repair(s), and the problem has not returned. It will now start and run fine in even the worst of conditions.
I now realize how important and critical proper fuel pressure is with these engines, and how an unqualified or overconfident mechanic could easily do an improper repair, resulting in fuel delivery failure at any given time, for any number of reasons.
Mods*/Admin* feel free to close off this thread if you wish. Thanks for all the help guys!