I would at least clean the IAC. If the valve is just getting sticky and thus sluggish it can stall the truck sometimes but still pass Tester’s test. Cleaning one is normally not very hard nor time consuming to do. And if it doesn’t help, count it as good “tune-up” maintenance. At most you’d be out a few bucks for some throttle body cleaner and a gasket. Of course an intermittent power problem to it could also do it, so I’d also take a good look at the wiring.
I don’t think it’s likely, but also in the realm of cheap and easy, I would give the brake booster line a careful inspection. One of the things that happens when you come off the gas is often a brief surge in vacuum pressure at the very moment you hit the brake. A small split in that line could become a sudden vacuum leak.
I would also think about temporarily pulling the vacuum line from the EGR valve and capping the valve and line. An EGR can stall a car (normally if you do it manually, though). This is also not high on my list of probabilities, but I wouldn’t put it outside of possibility.