Ok it's now 2016, but for those of you who still have a 2003 Tahoe, here's a story I want to share in case you're having the same problem related to this post but have not found a solution. My "Service 4WD" light came on intermittently over the past few years but would go away by itself. I had the transfer case serviced (oil, filter, etc.) and all checked ok. 6 mos ago, it started coming on again when I started the engine, only this time the chime started binging (same sound as the Low Fuel chime) and the binging would continue for about 10 seconds. When it did this, the A/C stopped completely as did Cruise Control. Driving down the street, especially when going over a bump, the A/C and cruise control would start to work again but the Service 4WD signal stayed on. When this started happening more frequently, I took it to my mechanic who said it was likely the ignition switch. This seemed to help at first, but a week later it started happening again. I researched the topic on the internet and saw folks were switching out the 4WD selector switch. They're pretty cheap and easy to replace so I did it. No change. The next symptom that started was turning the key and NOTHING would happen. I took it back to my mechanic complaining about the "new" ignition switch not working and the same problem still happening. He finally diagnosed it. It was the wiring harness in the steering column. He proved it by showing me when the tilt steering was all the way up or all the way down, it started fine. When it was in the middle where it usually is, it wouldn't start. He found a wiring harness for it and replaced it and now everything is as it should be. If your AC stops when the Service 4WD light comes on, try keeping your tilt steering all the way up or all the way down and check the wiring harness.
