Bouncing, lifting, jumping, and guessing are extremely poor ways to analyze and solve a problem.
Take your rig to a scale and weigh it. Feedmills, scrap yards, lumber yards, and quarries have them and will often let you use it for free. Or, go to a truck stop and pay the small fee. Then you will have some facts to work with.
There's no way your boat and trailer weight 6,000 pounds. I tow a 2004 23' Rinker bowrider on a tandem trailer with a 75 gallon fuel tank and dual batteries. With the gas tank full, cooler full, tubes, skis, etc the whole thing, trailer and all, weights 6,500 pounds on a CAT scale.
The shocks on your truck were crap from the factory. After this many years they are even worse. Replace them with heavy duty shocks. You probably have more tongue weight than necessary. See if you can slide the trailer wheels forward to take off some tongue weight.