omg how much misinformation can you get in one thread.
The spring controls ride height, and also controls how "stiff" the car is.
The dampers job is to control (dampen) the spring.
You DO NOT make a car stiffer by fitting stiffer (or turning up) shocks, all that you achieve by doing that is stopping the spring from doing its job.
If you fit a stiffer spring however, you need "harder" shocks to control the more powerful spring (which is why lowering a car on standard shocks can result in a very bouncy ride, the damper isnt powerful enough to control the lower, stiffer spring).
Similarly, the spring, and the spring alone, controls how much the suspension compresses when you add people or luggage.
If they are proper coilovers, they will be running a standard 2.25" or 2.5" spring, and these are readily available at motorsport outlets in a wide range of lengths and poundages. If they arent, then you're quite likely stuffed. Diesel model parts wont help you, as any increased weight in a derv is over the front wheels, and the rear springs will be no different.
Your only option is to get a set of custom coils made up to fit your existing mountings, but in the appropriate poundage for the application, and for the amount that would cost you, you'd be better off sacking off the crappy lowering kit and installing something half decent.