What might be the cause of a howling noise at or above 35 miles per hour it is not changed by shifting gears but sounds like it is coming from the center of the truck or the wheels?

There are several possibilities: the tires could be worn or have deep "lugs". If the tires have worn with "cupping" they could be making noise. If it isn't the tires, it could be the drive shaft or the drive-shaft carrier bearing if it's a 2-piece drive shaft. Another possibility is the differential. If the noise only happens when accelerating or decelerating it could be the differential ring/pinion gear alignment or wear to the ring/pinion gear.