01 Check Engine Trac Off Hard Shifting

[kok328]

Today my car (2001 Cavalier) starting shifting late and ran high RPMs before it would shift and when it did shift, it shifted hard.
Shortly after I noticed this problem the Trac Off light came one.
I stopped at my destination and turned the car off.
When I got back in to leave, I noticed that the car was still shifting hard and late. I also noticed that now the check engine light was on along with the Trac Off light.
I pulled into an Autozone and they scanned it for me and said it was a bad TPS. They also cleared the code and it seemed to run fine for a short while until the Trac Off and Check Engine light came back on and it started shifting hard and late again.

After looking through posts here, it seems that a bad TPS is not that simple of a conclusion.
I've seen post suggesting bad brake pads, ABS senor, bad battery, bad Air Intake sensor, O2 sensor, etc...

How can I know what's causing this problem and which part to replace?
Is the TPS trouble code a result of something else or the actual problem?