Is the speedo working? I would think if the drive gear or cable was broken the speedo wouldn't work either. You can always take it to a speedo shop for repair.
if i'm not mistaken..the odometers are set by the ECM in the late model stangs so even if you replace the unit with a factory new one it will go to the actual mileage... contact ford..i think that the odometer is covered for a lot longer than that of the original warranty... just a suggestion..
Mine broke on my 96 V6. I bought a cluster off of ebay for like 40 bucks and pulled the odometer out of that one and matched the miles and it has worked since.