OBD #740 code, speed sensor or torque converter?

My check engine light came on a while back, but the car showed no symptoms of an issue. I had my battery replaced and I thought the check light would stay off, and it didn't come back on until I took the car over 45 MPH. Other than not getting the usual good milage I've always gotten, she seems to be running as usual. I get a code on the obd #740 which comes up as a number of possible problems. I hear that the most common problem with that code is the torque converter. Is this true? How difficult is it to change either or both parts?