I believe the stock LJ rear spring rate is around 170lb/in, depending on what options you have. JK springs do not really fit. Nor do rear ZJ springs. You'll want to use a spring that was made for a TJ.
My best suggestion is to toss in a set of AirLift airbags. I had them in my LJ and they worked pretty well for carrying extra weight without making the ride too firm. I got five years out of the set I installed before one died. I didn't drill the hole in the lower spring pad big enough, and it would pull the hose off if I really flexed the suspension on the driver's side. If you do install bags, drill a 3/4" hole if you can. 1/2" is not quite big enough for the hose clamp, and can pull off the hose if it's not exactly centered in the hole.
You could also put a 3/4" spacer back there. That'll bump the rear up a bit, so it'll sit higher empty, but also higher loaded. (Sounds like your only complaining about loaded, making airbags a better option.)
Nth spring locators do raise the effective rate by ~5%. But they also lift it 1-5/8", which might be too much unless you want to bump the front up a bit also. I'm running them now, and am very happy with them.
OME 4162 rear springs will probably net you 1" to 1.5" on your LJ, but OME's spring rate is lower than the factory LJ rear spring rate, so overall ride quality may suffer and you'll still get noticeable sag when loaded. (Yes, the OME rate even for the HD spring is LOWER than a factory LJ spring!)
If you like your current height and ride, I would still recommend putting airbags in the back. Even just 20psi was plenty to keep the back of my Jeep from squatting when loaded, and at 5psi, you can't tell they're there. Capacity when you need it, and not when you don't.
Chris