I have a 2005 3.8 liter impala with 250K miles.
And at full throttle it will not accelerate it bucks and chugs and just won't go. Just like it has a clogged fuel filter.
When I put the pedal to the floor the scan tool goes nuts.
I'm thinking bad throttle position sensor (TPS). According to the schematic, it's a potentiometer. I suspect the wiper inside is making a bad connection when you go WOT. The signal from the pot will be erratic when the wiper hits the bad spot. Think static on an AM radio. That may explain why "the scan tool goes nuts".
Looking at my manual, it makes no mention of needing to do any sort of re-learn when it's replaced. So it's simply a bolt-on replacement. It should be easy.
For confirmation, I would put a voltmeter on pin B of the connector of the suspected bad TPS, and watch the voltage change as you move the throttle up and down. Then see what happens at WOT. If the voltmeter won't settle down, that would be my confirmation.
An old analog meter might be easier for this app, since the needle should move steadily with the moving throttle, until you hit the bad spot.
With a digital meter, you'll need to move the throttle to a position and hold it steady while the meter settles. Then move it to another spot and see the voltage settle again at a new value. At the bad spot, I would expect the reading to keep changing, to never settle.
In my driveway, I'd probably put a scope on pin B just so I can see the noise waveform, but that's not really necessary.