It sounds like the truck doesn't actually stall but drops to too low an idle speed. I'd be interested to know whether it actually stalls or if the idle returns to normal after a moment.
I'm suspicious that there's carbon built up in the throttle body. You can remove the ducting to the throttle body and clean the throttle plate and throat with throttle body or carburetor cleaner spray and a toothbrush.
If that doesn't do it, get it diagnosed by a GM specialist.
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