How can I be sure which transmission speed sensor I need to replace.

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The problems my car is having are as follows: 1. It won't shift into 4th gear. 2. Engine light came on. 3. TCS light is stuck on. 4. Gear position light flashes after I accelerate to 4th gear and slow down. Some good clues: 1. The lower gears work fine. 2. The speedometer works fine. 3. At about 55mph the RPM's start getting high and the car will not shift into any higher gear. Again, the lower gears work fine, always. 4. Cruise control still works. I have been told there are 2 sensors, input and output, how do I know which one needs to be replaced? AutoZone diagnosis comes up Speed Sensor. Which is the more common one needing to be replaced? Thank you for the help and input.