Hey boaterbob,
Did you find resolve to this issue? Seems my 2004 Denali may have this issue now, At 188,500 I was changing transfer case oil. Took out the fill plug and oil came pouring out. Besides the spillage when draining I measured two quarts and 20 ounces. I refilled to overflowing 1.5 quarts which is specification.
Then I checked the trans fluid (which was serviced two years ago at 150K) and its down maybe a quart and a half. That is a considerable leak and leaks nowhere else. I am thinking its leaking into the transfer case.