96 Mercury Sable shiftin or non excisting shifting!

My car will not "kick" into a higher gear after reaching approx. 40 kms. When it reaches 40 the rpm's will rev high and will not engage any higher. I recently changed a sensor on the passenger side abs because i had a abs light pop up on my dash. My abs light is still on and i am still having problems! My code reader brought up code# po500 with a speed sensor code.....i am unsure about the code's and feeling very frustrated!!:runaround: Any help, diagnosis, suggestions or prayers :iceslolan will be greatly appreciated!!!