I put the silver bilsteins on the rear last year to make pulling 16k fiver a little more stable.Don't remember the series but even with 3k pin weight rear tires come off the ground wile on the road.Empty you have to look for bumps in road befor you shift.Need some thing a little softer you hit a bump on the highway you can watch the tach jump up when you hit a bump.

 

probably the 5100 bilsteins, good stuff, but not much else will help except new leaf springs. those overload leafs are just too stiff. mini leaf packs from the likes of lorenz or carli help by replacing that thick solid leaf with 4-5 small leafs. Full replacement leaf packs or the lorenz dual purpose (good for towing) are probably the best option. otherwise you gotta keep a lot of weight in back all the time

 

Take the overloads off and put on air bags. The shocks can only do so much, I do not think double shocks would do anything.