Tire noise will be somewhat insulated from the frame or body of the truck, whereas drivetrain will be more directly causing a vibration that can be felt while holding the shifter, for example. See if you can feel the noise by holding on to something solid in the cab as you aproach 55 mph. This is basic vibration analysis. On my Discovery, I felt a vibration on the transfer case knob/handle, accelerated and let off the gas peddle all together several times, until I heard a slight clunking noise from the drive train, probably the transfer case. It needs to be pulled and inspected. My truck also has a growling noise at highway speeds, similar to what you described. Check for any signs of leaking fluids as well.
John O.
Crete, Ne.
