If you haven't already done it, do a search on 'brake switch'. There have been many posts over the last few years pertaining to the brake switch used on ML's which signals other systems to any depression of the brake pedal. Since the brake must be pressed prior to moving gearshift from 'Park', I'd check the function of this switch first. Others on this site may well have direct experience with the problem and have some suggestion as to how the switch can be ruled out.