The tranny uses a VSS (vehicle speed sensor)to change gears, amoungst other things. If the sensor has gone you loose your speedo and or your tranny will be thrown into confusion. It is mounted in the diff, once I had Fords do some work on the tranny only to find fluid squirting out of the speedo cable. It was traced to the oil seal on the VSS sensor. perhaps yous has come loose, seal gone / got covered in oil and packed up?
Worth a look.
