Wow, looks like there is a whole industry build around defeating the passlock. There are a few commercial devices like this one out for one , it even has a DIY plan for sale, that say they can defeat the passlock; GM will not disable it. the only other way I read about is the "White/blue" ;-)

"Step One: Start the truck, leaving it in park, and running during this process.

Step Two: Locate the plug pictured below, The plug is under the dash near the steering column. Open your driver door and stick your head on the floor near the gas and brake pedal and look up, it will be the big white plug staring you in the face. If you need anymore help let me know. You will notice that on the side closest to the front seats, the plug is pretty much split in two. We will be working with the right side of this plug. We are looking for the wire that is White with a BLUE stripe. It will be with other wires that are white/colored stripe.

Step Thee: Cut this wire in half, and electrical tape both ends.

Step four: Shut off the truck, You’re done!

Try restarting it, you will see the red security light on, but that can be removed with the LED mod how to." Check on here for more info. Hope this helps, good luck.

UPDATE

Images are from the same site as the link and credit to the guys at coloradofans.com as well as an interesting schematic that may offer more of a permanent solution.