P0502 Chevrolet - Trac off light on, shifting hard, speedometer is not working

Question

P0502 Chevrolet - I have a 2002 Chevy Cavalier that just hit 102 thousand miles and all of a sudden the engine started running at very high RPM's and the transmission is shifting really hard, the speedometer is not working, the Trac Off and Check Engine lights are both on, and the car makes a clunking sound shifting into gear. I see this seems to be a common problem with Cavaliers so I am wondering what exactly you think could be causing this. The Trac Off and Check Engine light are both on so I brought it in and had it scanned. P0502 is the service code that shows up and reads Vehicle Speed Sensor Circut low input, and check engine is showing a "bad O2 sensor". I brought my vehicle into a local repair shop and paid $120 to have the vehicle speed sensor replaced and they did some other voltage testing but it did not fix the problem. They said it was my transmission and wanted to drop it and change the fluid and filter for $250. I dont want to spend that kind of money just to see if the tranny is bad when I have not had any signs of it slipping or starting to go out. I did some research online and found that it could be a number of different problems that could be causing this. The TPS seems to be a common problem that I keep reading about and also the wheel speed sensors. I also read that faulty spark plugs could be the cause of the problem or a bad speedo cluster. Not sure what steps I should take next or what I should try first without spending a bunch more money and not fixing what is causing it. Any help or input would be greatly appreciated!