Shackles are bad. They overstress the springs.
Air shocks are bad. They unload the springs (leading to wheel hop) and put stress on the upper shock mounts (often leading to cracking).
You need to put in a different set of leaf springs.
Or, get a couple of new, longer studs (the ones that hold the leaf spring together). Unbolt the rear end and lift it off the springs. C-clamp the springs together and remove the center bolt. Take off the bottom two spring leaves (the shortest two. Flip over the second leaf from the bottom so it arches in the wrong direction. Put the bottom leaf in place and put the long stud through the leaves and tighten them up. The reversed leaf will lift the other ones. Cut off any excess stud sticking out the bottom.
This might work and won't cost you much.