I have a 99 Dodge Durango & there is a problem w/ the speed sensor.........?

The check engine light is on because the spedometer sticks and they pulled the code (I think it was P0500 or something like that) I am wondering if this is going to be a huge and costly fix or not. I guess there is something wrong with the speed sensor in the transmission? If anyone has had this problem or knows anything about fixing this I would greatly appreciate some advice. I need the vehicle to smog and it won't p if the check engine light keeps coming on!! Thanks!