I have an ML 55AMG - 2000 - whats causing the limp mode?

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Hello . I have an ML 55AMG - 2000 - its in limp mode - no reverse lights - the speedometer shakes at start up - the gear display is blank - and the shifter isnt coming out of park. The OBDII code I got is P0720. The car still goes into reverse but can't get out of the lower gear. I press the over ride with a pen to shift out of park.