No, you do not need a whole new transmission because it has a leaking seal.
Now, the fact that it doesn't want to shift worries me a bit. You may of ran low on transmission fluid and you have done some harm to your transmission. You will need to get it in to a shop so they can test your transmission to see if it is in good working shape.
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Monday, February 23rd, 2009 AT 5:27 PM