Kmagee: I have the same issue.. 2016 Duramax/Allison 12k on it. I have read here about it. Some chalk it up to "fuel slosh". It absolutely is not. I can't pinpoint it exactly, as far as what triggers it. But It never starts, right from a startup. It does happen, though exactly as you describe. Around 5MPH, when coming to a complete stop. It is a certain THUD, as if I get rear ended. The only thing that corrects it, is shutting the truck off. Usually after doing this, it goes away, for several hundred miles. I asked the dealer about it, they checked for bulletins, and they claim nothing, nor have they ever heard of it. They scanned my truck, but found nothing.. I am curious too, what this could be, and where this issue will be at 50/70/90k miles..