I have an 03' Tahoe which is doing the same thing. It has progressively gotten worse over the last few months.
My husband has an 01' Silverado which had done the same thing. First the dealer said it was a problem with the way the steering column was pressure punched in by the machine at the factory. The dealer then apparently re-did whatever the factory did incorrectly and it was fine for a while. Eventually (within 6 mos.) it started doing it again and we took it back in 2 more times. The third time they finally replaced the Rack and Pinion. It has not done it since. I will be taking my Tahoe in and expect that I will have to really stand my ground and fight for repair, especially since I was promised by the dealer I purchased it from that my truck would never have the same problem as my husbands since they were on different chassis.
Oh, and my Mother in Law has a 00' GMC Sierra Z71 and also has this problem. Unfortunately she bought it used and no extended warranty. Will GM cover that repair?