Buck.. I have swapped 04-08 transmissions into Impalas from GP's, Bonnevilles and even other GM stuff. The key has been to swap the diff. I've also been in your position of doing it in car.
Jack up just the driverside and support
Pop balljoint 18mm wrench
remove axle nut and axle 34mm/36mm socket
Remove engine to diff bracket 4 x 13mm socket or wrench
Remove other two 13mm for diff cover to come off
Hit diff pin/output shaft with hammer to split it off
Pull DS inner cv back and wedge something between it's cup and the trans (pulls output shaft out of your way)
reach in, pull parking gear out and the bearing that fell behind it.
Grease bearing to park gear and reinstall
put new diff with sungear in place and wiggle til it goes in
Sit cover on and ensure that it 100% percent sits perfectly flush and up against the trans case. (which is should since you gooped that bearing to the park gear)
Put cover back on
Put bracket back on
Put ps axle back in
Go to DS and remove your spacer
Give the DS axle a few shoves to seat the output shaft into the diff
Finish putting your passenger side together and smile that the code is gone.
