You can add helper bags to help support the rear and stiffen it up. You can install a better rear shock set also. There is helper laf spring dealio that you can put on your leaf spring all these items would help with towingIf this is not the right place for this, maybe the moderators can move it.
My 2006 GMC Sierra 1500 with 5.3 is used to tow our 25' travel trailer. The truck has 105K on it. It does a pretty fair job, but it there is some tendency to bounce. What suspension upgrades are suggested to help with towing?