The SDL has a relatively low idle speed compared to the previous vehicles that you would be familiar with. When idling at 600 rpm or so, the torque available through the torque converter is so low that it is not sufficient to move the vehicle forward on level ground. Naturally, if you are on a grade, the vehicle will have a tendency to roll backward. This is perfectly normal.

I tend to think of them in similar vein to a large jet airplane. You can start all four engines and leave all four running at idle and the airplane will never move.
Too damn heavy.