As a (former) GMC / Pontiac / Suzuki trained salesman, I'll throw in my opinion here. GMC and Chevy do have the same drivetrain and although parts on the outside can be swapped, they are different. Chevy is aiming for a younger demographic, with GMC going more for sophistication. You can order two identical trucks and the GMC will be about $ 750.00 more. However, you'll notice things like standard towing packages and running boards. Once you add these on to the Chevy, the price is the same. The one area where you'll lose - this is for you Chokey - is in resale. The GMC is worth about $ 1000.00 more at trade-in because fewer are made. Any time an item is made in fewer numbers, it is of more value. Thus, you buy the GMC. BTW, for every 10 cars I sold, 8 were Yukon / Yukon Denali models. And nobody ever ordered or wanted 4-wheel steering. It's just another drivetrain to maintain and drag on mileage, in the customer's opinions.
Also: The '07 GMC Yukons will be styled like the Cadillac Escalade but will be available starting in January/February '06!