97 and 98 use a "VSS" gear located on the tail shaft of the trans (on top).
I had to change mine to a smaller gear when I got taller tires to correct the speedo/odo.
If you pull off the cover, you should be able to pull the gear out. I think there is a clip on it too, so take that off too...
Check to see if it still resembles a gear and has all it's teeth..
If the gear is okay, it could be the sender itself or even something in the gauge cluster??
I've not heard of someone with a problem like this and I've been around here since 2001..