2000 Ford Ex Powerstroke w/ 114,000 miles - Loud rumbling between 75-80mph. Mild rumbling between 50-70mph. In both cases rumbling doesn't exist, or isn't that bad, while accelerating through these speeds. But, when it shifts into higher gear/overdrive at these speeds, rumble in the jungle begins. Problem began one week ago, suddenly, with no pre-cursors or red flags. When rumbling begins, steering wheel remains straight and does not shake. I balanced and rotated the tires, but had no change in the problem. It feels and sounds internal. I do a lot of highway driving(85% of mileage) with a fully loaded utility trailer and 5 passengers. Rumbling occurs both with a trailer and without...Any ideas?