2 Answers

Pull the front driver side axle. Sounds like the spline on the axle is stripped. Listen for noise when attempting to accelerate. If you have Dana posi rears they should both be turning the same speed under load. Put the vehicle on a sandy surface in four wheel drive and punch it. Both wheels should leave the same length trail. There are no clutches in a transfer .case 

My 98 Silverado does something similar when I am engaged in 4wd.  Not privy  to how the transfer case gives power to the wheels, as it seems to me a positraction front end would make it difficult to corner and steer properly and if one wheel is spinning faster than the other, it would be dangerous. Mine acts real stiff and there is no coasting to it, even in high range. I can drive it fine, but it is nothing like my 97 Bravada AWD which is seamless and I wish I could disengage it for gas mileage. How do the wheels behave when you are on a lift, it just seems to me that what you are describing is positraction in the front end, but is more like convention cars where the wheel with traction grips, allowing the other to spin for the reasons I stated. (at least in the front end.  I am guessing, as we may both need a better understanding of how they engage, which any good mechanic or manual will explain, I am sure. I have had Jeeps with Warn Hubs and if you forget to turn both in or out, all sorts of hellish noises occur.  Interesting question, wish I had a definitive answer, but maybe the auto hubs in the front end aren't engaging in sync.  I had a ranger and you could shift on the fly, but this GMC it has to be in Neutral or Park before it will engage.  I will follow this as I am curious myself.  Tires are all the same radius and style? Mine has 17" wheels and when I bought it the guy had increased the size, but left me w/o a spare.  The junk yard I went to had only the stock wheels, and they said it shouldn't be an issue for a short drive to get it fixed.  Just a thought.  Good Luck Woofer