Given real life experience with the same issue, I would have to say that airbags cure the rear sag issue and provide a little extra peace-of-mind when towing.
After much thought, I bought the AirLift 1000's from Amazon at about $80 and installed them in my rear springs. I ran the air line to the rear bumper and installed the nipple in an existing hole on the drivers side rear bumper. I have had these for quite some time now and love them!!! Highly recommended.
I find myself comparing the stance of my T4R to others when I see them in the parking lot. Looks much better with the correct rear support and having the option of stiffening or lightening the lift is a winner for me. I have found running about 16-18 poundsin them is perfect for my daily driving.