There was a hole worn in the transfer case causing loss of fluid and failure. This is a real safty problem because the truck will lock on you while driving and cause a serious accident. there is no way to prevent the problem even if you check the fluid level on a regular basis because a hole is created by a part that is inside the trasfer case that eventually wears thru the side of the case, causing total loss of fluidd This should be a recall issue for Chevrolet.