These 4th gen 1/2 tons are really washy in the back when loaded. I think it is primarily a function of the coil springs allowing the truck's weight to easily shift left to right as the truck rolls. The anti-roll bar would probably help, but don't expect any huge gains (I have one and still experience this problem). The other problem you could be experiencing is if the tongue is too light the trailer gets washy and causes sway.
If you are just towing a boat, and experiencing this problem I would be concerned. Do you only experience the problem after running over a section of particularly bumpy road? Is it possible you are new to towing and just not used to what it feels like if you are getting some trailer sway, and front to rear bounce that can happen after going over a lucky series of bumps?