Well, im sure Binary Terror could help you with this, he has a mammoth too. However, i'll point some things out for ya. You stated its a 2 speed. Your smaller spur at idle will probably spin freely, because that would be your 2nd gear spur. Which shouldnt engage until the mechanism reaches a certain RPM, then engages the clutch. Usually a pin style 2 speed. Now, as for the truck not moving when you give it gas, that cant be too hard to pin point. After all, there really isnt a transmission on that truck. When you hit the gas, does the power get to the clutch bell on the engine? If so, then move on to the 2 speed. Is the spur spinning when you give it gas? If so move on to the next obvious area, the diffs. Make sure your output cups still have set crews, or set pins still in them. Sorry i cant help more, i dont own a mammy.