2001 Mitsubishi Eclipse • 2WD Automatic • 198,000 miles

2001 mitsubishi eclipse, I'm having a problem with my eclipse. The car was running fine all of a sudden it got stuck in 3rd (it is an automatic) I went and had it put on the computer at autozone and they said it was the transmission speed sensor, when I bought it it didnt fit, I have taken it back three times already they now have it in research and has been there about a year now any ideas? I have been able to only find 3 sensors on the car I have the manual and it has not helped either. I just drove the car 800 miles for a trip and it made it just fine just running a little heavy, takes it a while to get up to speed. Any help would be appreciated. I just did a oil chage and chaged the spark plugs and wires.
March 21, 2011.

What was the trouble code code retrieved? Have you tried the junk yards for the part?

Mar 21, 2011.