any updates?

I would say if it only makes the noise in 4 wd then it would lead me to believe it is a 4wd issue.
You could try it on pavement to rule out the loose gravel contributing to it, just go straight, they don't like to turn in 4wd.

 

Does it have a the u-joints in a solid front axle at the hubs, or does it have the CV axles? Those u-joints are what usually makes that noise. My old ford did it. Its a pain to change on the ford, I don't know about the chevy.

 

U-joints.

The O/P hasn't posted for almost a year now so I'm assuming he fixed the problem by now.

 

Ha, I didn't even look at the date.