Essentially, as some have said, there are no immediate plans to make the "long" models as Hybrids. For now and for the next model year it is the Tahoe/Yukon and shortly there will be a Silverado/Sierra with the same system.
The main reason being the longer models tend to see a bit more heavy use, in the way that they do weigh a bit more in the first place and then tend to get loaded down with more people, cargo, and towing things more than the more commonplace short models--which get used by sole drivers as everyday "cars" more, in reality. More people buy a Tahoe/Yukon (or did, before reality set in) to replace a car, and just to be used by them for their daily grind, than the family&stuff-mobile long models.
Still, there's always a chance, and it would not be hard to put the system in these after trimming out the extra weight and the other exterior changes, but no plans anytime soon. It's an expensive system and will expand in use, but not immediately, and in limited form.