I second the rancho 9000 adjustables for towing, on long road trips I will turn my rear shocks up 2 notches and my front shocks up one to help handle the body sway from the boat trailer.
These are excellent shocks for every application, even better for people who tow.
If the rig is only used for towing you may consider the Rancho in cab controller, so you dont even have to climb under the back of the truck to turn the knobs, though I don't find this too difficult to do (then again my truck sits on big tires and is lifted)
They will not really stop the rear end sagging, but they will stiffen up the springs a little bit.
There are helper springs on the market or even airbags you can install to adjust the ride height with the heavy trailer.
If you have not already you should also add another transmission cooler or seriously upgrade your stock unit so you can extend the life and save your auto transmission
With trucks used for towing I also like to change the rear gear fluid and transfer case once a year, the transmission fluid and filter yearly or more also.
Cheap insurance, but then I keep my trucks forever..
