74 Duster advice please on raising rear 2 in

You can add a leaf spring to the current spring packs, have the springs re-curved by a spring shop, or buy new springs. Those are pretty much your options if you take air shocks and lift shackles off the table, which you're right in doing as those are just temporary band aids that will damage your car in the long run.

The problem with adding a leaf is that you're upping the spring rate too, so you may not like how the car rides. How much you add to the rate depends on the length and thickness of the leaf you add, and where in the pack you add it, so your results can vary. Re-curving is good if you have a spring shop local to you that will do it, but how long it works can vary. They're old springs after all. New springs are by far the best way to go, but, also the most expensive. ESPO (springs n things) sells good quality leaf springs at pretty decent prices. They also sell springs with different arches to them, ie, +1", +2" etc. If you go that route, remember that your current springs have sagged, so, you probably don't want +2" springs, as they'll likely lift the car 3" or 4" from where it actually sits now. You probably just want stock or +1" springs.