1993 F-150 XL - One rear wheel goes ping-ping-ping proportional to driving speed

Well, maybe not "ping" but "clink" -- Sounds like a piece of heavy sheet steel hammering onto another piece of steel. I could easily envision it as a heavy hinge mounted to a wheel, and every time of comes over the top of flaps closed again going "clink".

• It is a well-used farm truck that I just acquired, so it's been put under some serious stress in its lifetime, I'm sure.
• Happens in either 2 or 4 WD modes, and in any gear (including R, I believe)

Any thoughts? I've been tossing around "something stuck in the break drum", or "a loose bolt on the break mechanism", but am really not much of a gear head (this truck is my first foray into vehicle repairs beyond topping fluids, changing lightbulbs, and spark plugs)

TIA!