1995 Chevy Monte Carlo Could it be something minor or is it

Transmission problem
1995 Chevy Monte Carlo 6 cyl Automatic 192000 miles

My car has stopped shifting into gears. If I put it into reverse, the tires will noticeably change direction to move backwards, but the car won't more more than a centimete or two. It will do the same for when I put it into drive, overdrive, 1, or 2. I have been told it could be my speed sensor, but the piece auto zone gave us has only room for a two cable connection and the car has a three cable connection; also, the piece is backwards from the one on the top of the engine. My family said that might not be the speed sensor, and that the speed sensor may be elsewhere on the car, but I haven't been able to find a similar sensor anywhere. I have also been told I just need a new trans, but since they are so expensive, my family wanted to try minor things first. Also, the car has had problems starting, supposedly the neutral safety switch. I had to start it in between neutral and reverse a few times. To my knowledge, it has never (in the year I have owned it) started in neutral. I don't know if this is relevant, but the Service engine light is on because of an O2 sensor. Suggestions?