How can I get a part number and then find good used gravel guards that sit inboard of the rotors on the front axle of my '89 F150 4x4? Any leads or ideas appreciated.

 

Sounds like you're talking about the backing plate or dust shield. I know Dorman sells them. But I'd Google "89 f150 4x4 front rotor dust shield" for new. As for used, most salvage yards would use a description instead of a part number. If I recall correctly, the original Ford base part number is 2K004 R/H and 2K005 L/H and probably fit various Ford truck sins.

http://www.quirkparts.com/auto-parts...nt-brakes-scat