I have the same specs as you, and having the same problem. I'm 99% sure its in the front. The entire front vibrates under braking from highway speed (seat, wheel, shifter; I can watch the hood vibrate). Under light/Med braking from <40mph there is nothing. Warped rotary was my first thought. Truck only has 108k miles though, and rotate were resurfaced at 90k. Botched job maybe?