Ok, first of all, the way the stock shackles are setup, if you run a longer shackle you will lower the shackle since it runs a tension setup. A shackle flip does just that, it makes the shackle hang down instead of up. This automatically gives you 4" of lift. Heres the link: http://www.offroaddesign.com/catalog/shacklekit.htm
<font color="green"> Longer shackles on the rear of a K5 that has the stock configuration will actually lower the truck. The stock shackles are in tension, meaning the point at which the spring attaches to the shackle is higher than the point at which the shackle attaches to the bracket on the frame. Go out and look at your truck, and imagine it with a longer shackle, you'll see what I mean.
In order to lift the rear with shackles you would need to do a shackle flip, which switches the shackle from the original tension configuration to a compression configuration, meaning that the point at which the spring attaches to the shackle is lower than the point at which the shackle attaches to the frame bracket. In this configuration a longer shackle will give you added lift. The amount of lift is roughly half that of the increased length of the shackle. That is, if you get a 2" longer shackle, you'll get 1" of lift.
Now, by just doing the flip, you'll lift the truck, no need for longer shackles. www.offroaddesign.com sells flip kits that will lift the truck either 2" or 4", so the 2" kit would work great for you, although it would be a bit more expensive than just longer shackles.</font color>