Highly recommend Torklift Stableloads before airbags. You may find the airbags are not necessary. I have found airbags can unload the suspension. The airbags can carry some of the weight instead of vehicle suspension. The Stableloads engage the vehicle suspension right away. There are two versions. With your 3500 you may have access to either.
With Torklift product in the past I've seen various vendors have a sale. It seemed to vary month to month. You may find stuff cheaper by doing a quick search. One time I bought something from Texas, it was drop shipped locally to me.