Air shock maintenance

I have a 1997 Cadillac Deville, it has 150,000 miles. Overall the car is in excellent condition. However, like most Cadillacs with this body design the car is starting to sag in the rear. If I put heavy items in the trunk or go over a spend bump the car just drops to the ground as if it has no shocks at all! And usually the items that I put in the trunk are groceries. So it's obviuos that I need to repair the shocks. The problem is every mechanic act like they can't touch them, and tell me to go see a cadillac dealer. I have tried that but they told me that they only service the cars that they sell. I live in Savannah, GA and there is only one Cadillac dealer in town. Can someone tell me where to go to get this problem taken care of? That sag makes the car look so ugly!