ok, leg me tell you my experience on this issue, mine was sagging for a while until one day the pump died, a $1800 mistake because i should have fix the rear shock air bags... once fixed it sagged again after a few days, went back to the dealer and wanted another $1000 to fix ($500 for bags and $500 for labor) i was all ready broke with the $1800 so i investigated and you can actually change them yourself...

you can buy the bags from Air Suspension Pros | Find Quality Suspension Lowering & Lift Kits for around $90 each, both are the same part number..

to replace you can simply wait form the vehicle to go all the way down, then lift and remove the tires, do not open the doors as the pump will start and refill the bags and you dont want this to happen... once you remove the tires you simply hand twist and pull out the old bags, remove the pipe that feeds the air, pick up new bag, install the air pipe, place in, twist and lock, you will do the same on the oposite side, re install wheels, lower the car "slowly" as you want to make sure that the installed bags does not come out of place, then start the vehicle and they will fill up and you are done, will take you around one hour and you dont need any other tool that you all ready have on the tool kit that comes with the vehicle.

hope this helps.