I found that GM is unsure at this time what is causing this to happen. The most commonly diagnosed component is the shifter, but they are finding no problem with the shifters when they are returned to the factory for analysis. I will attach an engineering investigation that I found that may help you. You could try following all the steps and see if anything jumps out at you. If you find anything that doesn't work the way the document states, let me know and I may be able to help.
If you are under warranty still, it may be best to leave it the way it is so a GM dealer can document all the diagnostic steps they are calling out. This may or may not help the factory figure out what is going on. I know it's not your job to help GM fix the cars, but for a good repair, it may be the way to go.
I'll attach the document to the bottom of this answer.
Hope this helps you get things figured out.
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#08-07-30-007A: EI08045 - Key Cannot be Removed from Ignition with Gear Selector in Park, Shifter Cannot be Moved From Park Position (Engineering Information) - (Apr 8, 2008)
This bulletin is being updated to revise the diagnostic steps (Steps 9 and 10) based on the technician feedback received to date. Please discard Corporate Bulletin Number 08-07-30-007 (Section 07 - Transmission/Transaxle).
Some customers may comment on either of the following conditions:
the key cannot be removed from the ignition.
the transmission gear selector (shifter) cannot be moved out of the PARK position.
GM Engineering is attempting to determine the cause of not being able to remove the key from the ignition and/or the inability to move the transmission gear selector from the PARK position. Analysis of the shifter assemblies returned through the Warranty Parts Center show NTF (No Trouble Found). Therefore, GM has a need to obtain information from the technician during diagnosis and PRIOR to repair. This information will be used by engineering to root cause the condition and develop/validate a field fix.
Complete the steps listed in the appropriate section based on the condition. Note all information on the repair order.
Important: Cable re-adjustment is easier with two people as one can hold the shift lever in Park during adjustment of the cable.