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! >Has anyone experience unbearable wind noise inside the cabin when both rear windows down completely? I'm thinking since there's no place for the wind to go and the back window is right there (no opening) you'll hear the loud noise. Any other ideas?
Do you have a tonneau cover or cap? They also might smooth out the air flow.