What kind of tracking? From your general description I would suggest that you track using Mocha AE? If you are using Corner Pin tracking and your shot is shaking then the track is shaking and you're probably not selecting enough area to track. If that is your procedure then you have to make sure the track is accurate before you apply corner pin. The other option wold be Camera Tracking which is not the right tool to use for screen replacement.
More details will help, but the suggestions I gave should get you started.
thanks for the help. I'm not using mocha AE because it's going to cost some money (I'm not doing professional work or anything yet). So yes I am using the built in corner pin to do a 2D track of the position, roration and scale. But, I will try enlarging the boxes even more to see if that helps
mocha AE is included for free in all versions of After Effects since CS4.
Here is a tutorial where I show how to do an advanced screen insert with mocha AE and After Effects:
Here is another one showing how to do it with a green screen on the computer screen
These tutorials use my script MochaImport+. But all the mocha tracking, keying and compositing works the same also for mocha workflows without MochaImport+. If you are still learning, note that we offer for all mamoworld products an education discount.
And if your track is still shaky when tracking it with mocha, here is a tutorial showing how to fix shaky mocha tracks
mocahAE is part of every AE install, so nothing is lost by trying nor does it incur any extra cost. The rest would require to see the footage.