Yep, the GP has Magnasteer and the Impala and Monte don't.I wasn't aware the 97-03 GP had variable steering. If so, I don't believe the Impala had it, and I'm not sure.
With everything coming out (engine and trans) this would be the ideal time for me to upgrade if such a thing is to be had. So Im kinda watching this thread with much anticipation.
If you don't have Magnasteer, there's no reason you can't swap out with a Impala LS/SS rack.