Your '99 Concours has an active suspension (electronic)
the OEM replacements are pretty expensive
I can sell you the OEM parts for a bit more than you can buy the aftermarket (monroe) active replacements
the front struts have no air
the rear shocks have air leveling assist (ALC/ECL) - normal shocks with an air bladder to adjust the height
the air bladder is inflated/deflated based on how much weight is in the rear seats and trunk to keep the car level
this keeps your headlights aimed correctly and makes sure that the traction and stability control can still operate correctly
you can convert the vehicle to a base/d'elegance suspension (passive, non-electronic)
it's not super hard to do but you have to make sure you disclose the modification when you sell the vehicle as potential buyers might expect the elctronic suspension
there's tons of info on here about the conversion from active to passive and the part numbers for both
These cars do not have air suspension, they have coil springs front and rear, oil filled struts in front, and oil filled shocks in back. There is a rear load leveling system (ELC), that inflates air bladders that are integrated into the shocks, to maintain proper ride height. If you install non ELC compatible shocks, the rear will sag with any additional weight in the vehicle.
The term "gas shock", or "gas charged" has nothing to do with ride height. Shocks/struts are partially filled with oil, the rest is air. Over rougher surfaces, the air and oil can mix, decreasing the effectiveness of the shock/strut. Removing the air, and replacing it with a pressurized gas, reduces the chance of the oil getting aerated.
Normally, when at least two people ride in the backseat the rear seems to bounce more. If it's travelling at normal doped limits and hits a bump the back end hits the ground. I'm not sure if the ECL is working properly, but there are no PCM codes.
Use the sticky post ^^^ "How to pull codes" section for your car, year, model. It's all built into your car. Write down all found codes and whether each is Current or History. Return to the sticky and open the link with "obd2" in the address. That takes you to a Master Index of all P,B,C, and U codes. Decipher all your codes and post it all here.