Correct me if I'm wrong but air shocks do the same thing as rearlift shackles right?

 

To a certain extent, yes. Shackles just push the back of the truck up without changing the spring rate so your ride will not change, nor will your load carrying capacity. Air shocks add spring rate (load carrying capacity) which is what lifts the rear. The ride quality will suffer if you try to get too much this way.