If the "D" light is blinking that is the factory indication of an electrical problem with the transmission. The transmission vehicle speed sensor is the input for the speedometer as well as for the fuel injection control system that determines when to upshift and downshift the transmission.you can have someone "pull the code' from the onboard system to confirm the problem is with the Vehicle Speed Sensor (VSS) but from your description it almost certainly is.there are several places that will plug in the diagnostic scan tool and pull the code for free. among them are AutoZone parts stores and AAMCO transmissions. Stop by one on your way to work or on the way home and ask them to pull the code for you. then you can be sure instead of going on a spending spree buying things on guesses. If it is the VSS as I guess it is, its easy to replace.hope that helps