You need to work out if the compressor (up by the chassis rails under the passenger seat) is running all the time to keep the air springs up to pressure - cos it shouldn't be !
If it is, then you've either got an air leak between the compressor and the springs (but I think it's 2 separate air lines), or the compressor is stuffed.
If the compressor is not running all the time, and only cuts in to keep the truck level, then it's probably the springs themselves. The way to check is to start the engine, shut the drivers door (SLS only works when the doors are shut), then stand on the rear step and bounce up and down - you should hear the compressor cut in.
Another thought - the SLS sensors (thin flimsy plastic lever things on the trailing arms) may be playing silly-beggars, I think you can take them off and 'work' them to 'clean' the potentiometer tracks.
Jim