All springs sag over time depending on the load they see. Obviously a spring that sees more load (drivers side) will sag quicker than one that see less load (passenger side).
This isn't a problem or defect with springs, it's an issue with weight distribution, specifically the fuel tank (and driver) that always loads the drivers side up way more than the passenger side, constantly.
If there are no other causes/issues (be sure to check your body mounts and that EVERYthing related to the leaf springs is TIGHT- as in torqued to spec), then all you can do is compensate for that eventual sag by swapping the leaf packs left to right. Eventually the springs will both reach a static height where they won't sag anymore.
If this is too low and you want a bit more lift in the rear to match the front, you can get shackles that are slightly longer (Warrior makes them as WAR123s), which doesn't sound like much, but they are perfect for lifting the rear about an inch. If you want still more James Duff makes shackles that are longer and should lift the rear 1-1.5 inches. WAR153's are 3 inches longer and of course lift about 1.5-2 inches. You can also just get cheap universal shackles at the parts store or make your own from flat steel.
I wouldn't use an add-a-leaf, especially not just on one side, which is a common suggestion for a fix. Swap them side to side, compensate for sag with slightly longer shackles if needed. Goes for worn down stock springs or aftermarket.
SOA will lift the rear 5.5-6" with new springs, but might be more like 4.5-5" with sagging ones. WAR153's would add another 1.5-2". If you swap them a few times and the rear end sags way too much, a SOA might be just the thing. You probably wouldn't need the Warriors unless you go more than 4" in the front with spacers or the 5.5"-6" springs.