I'm wondering if you're experiencing the same issue I had with my 2wd 05'.
If so, and I would go ahead and check this anyway. But, there is an apparently common issue with the bolts securing the front cross member, backing out a little.
The noise I was hearing was the crossmember ever so slightly moving, causing what sounded more like a "snap", or even a creek from the ball joints.
I say this is a common problem only because when I described what I was talking about to a local service writer at the GMC dealer I bought mine from, he was quick to suggest this very issue.
Sure enough, the bolts were just a little loose (meaning the really needed to be further tightened... they were already "snug"). Since they were tightened, no noises!
I would suggest, if you have the means, put the front end up on a floor jack and check the bolts. Actually, I would prefer you put the vehicle on jack stands, rather than getting under it while its just on a floor jack. Don't forget to check the vehicle before getting under it! Make DARN sure it's secure!!!