Guys...found this post...a bit old, but highly relevant to me. I have a 2002 olds Bravada (newest model, first ones off the assembly line). We were on vacation in SC (650 miles from home), and the front differential started making a horrible grinding noise. Gets especially bad when braking or slowing down. I've never seen a leak in the case, but is there a recall on anything in the front diff? Has anyone had this sort of issue? I suppose that it's possible there is a crack, and the leak has been really slow, leading to an empty case now. The car does spend a good deal of time away from the garage, and there is that big crossmember under it.
Also, as far as the transfer case issues, I switched to a race-grade Red Line auto tranmission fluid in the casing. I'll have to look at which fluid specifically, but I haven't had any binding or other issues with it in 50,000 miles. Screw that GM stuff.