I have a 2000 Yukon XL with 153k. It has the 5.3 Vortec. Great running truck. On the way home last night, the 'Service 4WD' light came on. I checked the manual, it said that there could be a problem...nice. Did a search of the forum, did not see anything posted. Truck is a family mobile, not used hard, highway miles. Changed rear diff fluid 10k ago because the pan had started to rust thorough and was wet.