Transmission troubles
 

A week ago my check engine light came on and the car began to have problems shifting in the correct rpm ranges. the code i pulled was for the throttle position sensor. as of yesterday the only gear i have available to me is 1rst. the trans has been recently rebuilt. am i right in replacing the sensor/throttle body assembly or is there some other common issue i have overlooked before i drop the $600 on this sensor?

How recent is recent? :)