load caring

I have been looking for a set of air shocks for the rear of my 96 ram 1500 4x4 but can find nothing listed for my truck. Is there a reason that they are not offered? and/or has anyone run into this problem and found a solution?

I pull a trailer 5 days out the week and having problem with the ass end sagging is there a load caring shock or even a heavy duty shock that will fit my truck.