The suspension issue is most certainly to do with leaky air shocks or air bags. What you'll need to do is find out what system the car has - rear only load leveling was standard, full air suspension was optional.
If the car has the rear only system and the shocks won't hold air, you can convert to standard shocks and springs without too much of a cash investment. The rear springs need to be swapped to either Dynasty springs or new variable rate springs, and then you can use normal shocks. While you can do the shocks without doing the springs, the rear end will bottom out easily.
Retrofitting the full system is a much more involved job, but also possible. You'd need to swap the rear springs, shocks, and axle beam (the air bag system uses different spring mounts IIRC) as well as both of the front struts.
From the pics, I don't think this car has the full air suspension. The saggy rear end is most likely due to leaky air shocks. Often the compressor will burn out from running too often... when you look at the car, start it up and listen to hear if the compressor comes on (you'll know when it runs - they're not that quiet). Then see how quickly the rear end comes up, if it does at all. Sometimes, you can get away with replacing the compressor and the shocks will still hold enough air to get by for a while. The retrofit to standard shocks and springs is inevitable though - you can't get the right side air shock at the dealer anymore (and it was profoundly expensive when you could).
I should also mention - if the air shocks are bad enough, bottoming them out can get them to seize. The ride won't be pleasant if that happens.
