I rotated the front two that were pretty wornout with the back two that have a pretty good amount of tread left and WOW It feels like a brand new car again. It used to squeel tires in curves at about 30-40mph now I could take curves doing 50-60 and not a peep or a feeling of sliding.

But my question is.... Now that I did the rotation the tranny ISN'T acting up like it used to...

It used to be slow shifting and sometimes feel like it was slipping. All in the same drive most of the time. I took it to the tranny shop and had them test it. They said the code siad it was "Incorrect Gear Ratio"
Well now that I rotated the tires it seems to be doing it ALOT LESS.

What could be the cause of this? Could it be because the tires have more tread, giving it a closer ratio to stock? Or am I wishing?

The only thing i can think of here is the wheel size or tire size. Are the tires all the same size? are your rims different at all?

All the same that I know of. All steel wheels and 215/65-15 tires all same brand and style too.

Your tires must have been REALLY worn (and/or badly underinflated) if the PCM was reading them as the wrong diameter.

Are you sure these are the original wheels for your car? Might it have originally had 16'*?