2000 Quest Speedometer Problem

I have the same problem a number of people have posted on here. My speedometer is now unstable between 0-40 mph. It bounces all over the place and I know if I don't fix it that it could damage the car because of shifting issues. Based on what I have read I'm going to go ahead and replace the speed sensor. Can anyone give me any clues/hints as to where this is at and how difficult this job might be? Thanks.