Are you using a UCF20/21 motor / trans? These dont have a separate vehicle speed output. The early boxes have three speed sensors , Input shaft speed , output shaft speed and vehicle speed. If I use a 20series box I often change to a 10 series tailshaft with the extra sensor. As for your changing it sounds like your box is in failsafe and locked in third gear. You will have to do some diagnoses but I would suggest starting with the electrical side of things. I would check for fault codes on the motor and gearbox. From there check the shift switch has power and has outputs when you move the shifter , scope both shaft speed sensors and then check the solenoids have power then receive a signal from the ecu. I will check my diagrams for the speedo output. I also think I have a spare trans tailshaft if that will solve your problem. Cheers