The transmission is electronically controlled. I wouldn't get too humg up on that, it is pretty normal for the vehicle to want to maintain the highest possible gear for the longest amount of time. By doing this it helps efficiency. What if you manually downshift it into 1st when you are at around 10-15 mph approaching a stop, then will it stall?

It definately sounds like an idle speed control problem. I am still convinced it is a dirty throttle body. I assume these trucks don't have idle stepper motors since they are drive by wire.