As GM continues to get its financial situation under control, the company is also evaluating ever model to determine whether a replacement should be ordered. In the case of the Yukon, there has (so far) been no replacement in the works. Don't despair just yet if you're a fan. GM could easily continue building the current Yukon on the same platform with regular exterior refresh updates (much like it has done on the Savana full size van).
This would allow GMC to have a full size SUV in its product lineup and as long as Ford continues to produce its full size Expedition, the Yukon 2WD is a necessary vehicle for GMC.