I'm leaning toward the D or dual motor contactor. It should pull in when you plug. A good way to test is to put a cable on both sides of the contactor to simulate a closed contactor, then drive the truck in a straight line. This truck is not going to make turns on one motor with the cable on there, but it will tell you if a D contactor is the problem. Contactor will close, even at very low speeds, when plugging if it is working correctly. It must have good tips also. Don't rebuild it, replace it, if it is bad.