If you got hit in the rear check the wire to the speed sensor in the differential.

If you think it's the instrument cluster, you can order one off ebay for ~$75. However, you need to make sure you only buy one with the same color back and if you want your mileage to be the same you should look for a cluster with the miles stated on the ebay auction; i.e., your new cluster will read out whatever mileage is stored on the coding chip.

Also, don't turn on your key with the cluster out of your car or you will trip your SRS light.