It's very difficult to answer your question since there is a lot of variance between vehicles. It really depends on how much your springs have sagged. If you want a level ride, you are more safe to go with the 1" shackles since most of the 2" that I have installed still ride a little high in the rear.

The 3 Add A Leaf system is a MUCH better solution for raising the rear of the truck, (since you get better suspension) but you WON'T get a level ride unless you replace your upper control arms and then adjust your T-bars to the max.

You mentioned that you are going with 31's. You can adjust the front and trim and not do anything with the rear for now if money is an issue. The shackles aren't needed for putting on those tires.

I hope this helps.