I tried:
a) Spray undercoating under the rear floorboards and hump area. Minimal help, prob not worth the effort.
b) A cheap walmart cargo mat cut to to fit under the seats and the gaps behind them. Helped some, with the benefit of stopping some heat in summer
c) Some old Moose brand heat shielding in the area under the dump bed. This was primarily to help quell the diff noise which is one of the biggest sources of noise at certain speeds. Helped a bit in that regard.
d) As an experiment, i put an old sleeping bag i was carrying anyway (for the kids...gets cold at night in the desert regardless of how warm it is during the day) up in the top, held up by a spider bungee.
To me, this is probably the biggest bang for the buck. I never got the chance to do it, but some acoustic foam glued to the top would probably be the biggest reduction you could do. Then maybe some around the engine bay (mind the heat).
Pull the windshield when you don't need it. Even a 1/2 shield reflects a little noise. If its bugs, get one of those screen windshields.
Most people know this, but it amazes me how many people run around with the rear seats up and complain about noise. Put them down unless being used. A cargo mat back there helps too, when not in use.