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If you fluid is full and the fluid looks and smells normal it is just going to be the overdrive solenoid.
It is not very expensive but the trans pan does have to come off to change it.
Yes on the valve body.
That should not be the problem on that one, Here is the description of what the speed sensor does.
This sensor is located in the extension housing of the transmission (2WD), or the extension housing of the transfer case (4WD), and it generates 8 voltage pulses for every revolution of the output shaft. These pulses are sent directly to the engine controller. These signals are used to determine distance traveled and vehicle speed. The engine controller also uses these signals, along with the signals from the Throttle Position Sensor (TPS), to determine the difference between a closed throttle deceleration condition and a closed throttle idle (vehicle stopped) condition. Under idle (vehicle stopped) conditions, the engine controller receives a signal from the idle contact switch and it also sees that there are no signals being generated from the vehicle distance (speed) sensor. The engine controller adjusts the Idle Speed Control (ISC) actuator to maintain a desired engine idle speed. Under closed throttle deceleration, the engine controller receives signals from the idle contact switch and the vehicle distance (speed) sensor, because the vehicle is still moving, and it adjusts the ISC actuator to maintain a desired MAP value.