Wether they are independent of each other depends on how the were installed. Proprely they should be tied together.
Most importantly is what the heck an unladen truck is sagging for? Start by putting on proper new shocks. If they do not get and keep it back up where it belongs, then you have another problem. Do you know the history of this truck, how it was used? I suppose that it is possible that someone destroyed the springs,the mere fact that the air bags exist point towards overloading and abuse.
I installed these on my '13 instead of bags.https://www.tuftruck.com/product/ttc-1211-rear-suspension-coil-spring-kit/ In one step I increased the capability if not the RATING of my truck