Have you checked the sender assembly on the rear axle? The unit is pretty exposed under teh car and has a tendency to be corroded if the car is an older model (say 10+ years).

I was under the car fixing/replacing my fuel sender and saw that my speedo sender looks liek garbage. Still works at 17 years though, so I just avoid touching it at all.

You might also want to check the wiring from the speedo up to the dash. It runs beside the fuel sender wiring through the trunk, and has a connector on the backside of the rear driver's side wheel well. Maybe the connector there is loose.

If teh connector isn't loose, the wire from the axle to the wheel well is secure, and teh sender itself is fine, follow the wiring up the driver's side of the car just to make sure. It comes up and in at the front of the driver's door frame near teh driver's left foot I believe.

If everything else is kosher then it might be the speedo/ODO reader itself, although teh failure of all these components leads me to believe it is either the sender assembly or the connection at the driver's rear wheel well.

goog luck,
rick