I installed add-a-leafs on my 89 STX. It was not very difficult to do. I didn't care much for the provided straps that wrapped around the leafs to keep them aligned. They were weaker than the stock ones and I had to readjust the leafs a couple time after years of use off-road. They did work though.
I had a pretty heavy shell and carpet insert on the bed and the STX sat high in the front with the factory lift. The add-a-leafs leveled the truck nicely with just the shell on it. It also road nice with the shell on it. With the shell off (which wasn't very often), the rear was very stiff and not a comfortable ride plus the rear sat high.
When I went off-road, it would be for a considerable length of time and I had to carry a lot stuff. The rear would sag again with that load and I ended up installing air bags to level the truck with a load and for towing. This worked so well that I skipped the add-a leafs on my next Ranger and just installed the air bags. Add-a-leafs are not adjustable, but air bags are and can be set to level the truck no matter the load, or no load, within the Rangers capacity. Air bags have some drawbacks too but I have learned to live with that.