Very well could be brakes. Caliper hanging up slightly will cause this. It may not be bad enough yet to cause a problem going forward. Also check to be sure the parking brake is fully disengaged.
Does this happen when you first get going and the truck has not been driven in a few hours, or is happening all the time when in reverse?
I have noticed on my truck that the brake rotors rust fairly quickly when it is parked in a damp area for even a just a day. The rust comes off after a few applications of the brakes but it makes a high pitched scraping noise until it does come off.
If the noise gets worse or happens while going forward you may need brakes. Your truck is old enough that I would not be surprised if you needed the brakes serviced or replaced.
My 2008 did something similar. When backing up it made a hollow ringing sound, not a grinding sound. Almost like rubbing your wet figner on the rim of a glass and getting that weird sound. Turned out to be sticking pins on the rear calipers, made the rotors ring. Only did it in reverse. Removes the tires, removed the rear calipers, relubed the pins with a high quality greas, not silicone. Never made the sound again.