is it a buffeting that you are hearing or the whoosh (roar?) of the wind rushing by? I have have buffeting on several vehicles over my driving lifetime and some were unbearable and I have also had the wind rush blast in the driver window so strongly that I had to keep the windows up! Does it do it with the front windows down? You could try the window vent/rain guards, I don't know that they will definitely help but they could change the aerodynamics of the truck enough to stop the noise somewhat...maybe, no guarantees! >
Do you have a tonneau cover or cap? They also might smooth out the air flow.