It could be several things such as the vehicle speed sensor or the transmission control module so you need to get the trouble codes first. There is a procedure to get the trouble codes on the transmission. You can cause the "CHECK ENGINE" light to blink the error codes. Without starting the engine, turn the ignition key on, off, on, off, and back to on.
The "CHECK ENGINE" light will flash the first digit, pause and then flash the second digit of any stored error. Example: one flash and pause and eight flashes = number 18 error code.
And the possible transmission trouble codes are:
11 No Engine Speed Sensor Signal
12 No Output Shaft Sensor Signal
13 No Vehicle Speed Sensor Signal
14 Governor Pressure Sensor Signal
15 Throttle Position Signal
16 Transmission Temperature Sensor
17 Overdrive Off Switch Circuit
18 System Voltage Out of Limits
19 Standby Voltage Out of Limits
21 Governor Pressure Solenoid Circuit
22 3-4 Solenoid Circuit
23 Torque Converter Clutch Circuit
24 Overdrive Off Lamp Circuit
25 EEPROM Checksum Error
26 Governor Pressure Sensor Offset Drift
Once you have the codes, come back and we can take it from there. Good luck!