Quite possibly a shifter linkage bushing problem causing the linkage to bind. Is on the side of the trans up high directly below the shifter area. Mines green.
See #27 for location.
http://www.detali.ru/cat/oem_mb2.asp?TP=1&F=208470&M=113%2E943&GA=722%2E632&CT=GA&cat=23Y&SID=27&SGR=095&SGN=09
Is also possible you've gotten debris in the shifter or that the shifter position sensor is at fault.
It is not in limp home mode (first gear only) right?
Posting the year in addition to model will help in future postings.