The D2 Td5 auto only locks the torque convertor under specific conditions when descending. It requires low range, 0% throttle and revs above 2000rpm, so only kicks in when the speed is above 9.5km/h which more than what I'd consider to be comfortable for a steep descent. It's mainly designed to work as part of the HDC system to assist slowing the vehicle when road speed is above target speed. The minimum (forward) target speed for HDC is 7kph. For crawling you have "driving through the brakes".
cheers
Paul

