There is also a TSB for your vehicle it is a little different than what you described, but it may still be the cause. Here is the TSB.
Bulletin No.: 06-02-35-005
Date: February 28, 2006
TECHNICAL
Subject:
Key Stuck in Ignition, Key Will Not Return to Off Position, Shifter Button Won't Release in Park Position (Adjust Shifter Detent Finger)

Models:
2002-2005 Chevrolet Monte Carlo

Condition
Customers may comment on any of the following conditions:
- The key is stuck in the ignition cylinder.

- They cannot return the key to the off position to remove the key.

- The shifter button won't release in the park position.

Cause
This condition may be caused by the detent finger shifting over time. This may result in mis-alignment and binding of the shift rod on the shift position guide.
Correction
DO NOT REPLACE THE SHIFTER ASSEMBLY. Use the following procedure to adjust the shifter detent finger.
1. Apply the parking brake.

2. Shift the transaxle shift lever from Park to 2.

3. Remove the front floor console trim plate screw located between I/P and shift lever

4. Open the storage compartment door.

5. Lift the trim plate up and slightly rearward.

6. Remove the wiring harness from the retainers.

7. Disconnect the traction control switch electrical connector and disconnect the heated seats (if equipped).

8. Loosen the shifter boot retainer and rotate out of the way in order to gain access to the shifter detent finger.


9. Loosen the fastener to allow the detent finger to adjust.

10. After the detent finger is adjusted, tighten the fastener.

11. Connect the traction control switch electrical connector and connect the heated seats (if equipped).

12. Install the wiring harness to the retainers.

13. Reinstall the shifter boot retainer.

14. Reinstall the console trim plate and screw.


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