If you do not want to increase your track width so that your front tires stick out more than your rears, than you need a full drop kit as opposed to a knuckle kit. Procomp is a popular 6" full drop kit. Do a search for "full drop" here or on any other GM forums and you will get a wealth of information. A full drop allows you to retain your stock wheels, which is a plus for some people who love their stock rims or want to keep costs down. There are knuckle kits that allow you to retain stock wheels, but their lift is usually limited to about 2-3 inches, such as Skyjacker's kit.
As for the lower rear end...Your truck sits about 2 inches lower in the front stock. People that dont want to lift their trucks, but want a level stance, either purchase leveling kits or crank their stock/aftermarket keys.
If you want to raise your truck evenly, the rear lift will usually be 2 inches less than the front.