Ball joints, tie rods and end links will make a banging noise if bad, not a grinding. A axle makes a clicking if the outside joint is bad and a banging if the inside joint is bad, I believe. If it happens at real low speeds, have someone stand outside the truck and put it in reverse and cut the wheel and have them go to all four corners of the truck and see if they hear it more in one place.
As far as what it could be without being there I would have said front hub, mine went bad and it sounded like it was a rear end bearing when first starting to go bad. When it got real bad and was changed the next day cause it was so bad, it sounded like it was the front pass side, it turned out though to be the drivers side! I guess check the pass side and or I would check the driveshaft for movement, my one buddy has a bad pionion bearing in the rear diff. I would also check and remove caliper up front and inspect both inner and outter pad, caliper could be bad and not releasing and wearing pad down to backing plate on the inside pad and that could give you the grind noise. Good luck keep us posted!