Check your blinker fluid...
Seriously, it could be a few things. Does it sound like its running rough or anything, or is it just a lot of vibration? Could be anything as simple as a tune up to something like motor mounts. Also, please list in your post what type of engine you have (4 or 6 cylinders). The 4 cylinder engines are notorius for having a bit of vibration at idle, normally only in drive though.
Please, I suggest you search and see what others have done in similar situations to yours. There are usually 1-2 new TN'ers every week it seems asking this very question.