When you change the rear differential gears to a different ratio, the speedometer will be thrown off. However, if the odometer is reading blank, the SES light is on, and the speedometer drops to zero, you sound like your having an electrical problem. If the SES light is on, I recommend going to autozone and getting the codes pulled. That will help to pin-point the problem. As for the speedometer and odometer, I would check and make sure no one was screwing with your electrical system and instrument cluster.
Just what all exactly did you have done in this "gear upgrade" ?