I have a 1993 Ford F-150 with the 5.0L engine, E40D transmission and four-wheel-drive.
The harmonic balancer is rattling loose on the crankshaft end. Is it pressed on? How do I remove it? Is a used part from a wrecking yard an option? And if so what other years will work on my '93?
Ford's 302 C.I.D. (5.0-liter) V8 harmonic balancers (torsional damper) use a retaining bolt and are press fit to the nose of the crankshaft. You'll need a special removal tool (puller) to get it off, and the correct installer to put it back on. It's also recommended that the new damper be placed in boiling water and then lubricated before installation. This expands the inside diameter of the damper hub so it can be pressed onto the outside diameter of the crankshaft nose with minimal force.
Some torsional dampers, like on a 5.0-liter Ford, do actually help balance the engine externally. But its original function was to dampen the twisting of the crankshaft caused by the pistons' downward force following combustion. This prevents damage to the crankshaft and other timing related components.
The big question is where exactly your torsional damper is "rattling loose". It's comprised of a center hub and an outer ring separated by a rubber cushion (where the damping takes place). If the rubber cushion has deteriorated and the outer ring is riding off of the inner hub, it's no big deal. Replace the balancer. But if the inner hub is loose on the nose of the crankshaft, replacement of the crankshaft may be required.
If you can find the correct damper in good condition at a "boneyard" (automotive recycling center), by all means buy it. They usually last a lot of miles. There are a number of different units that will fit on the end of your crankshaft, but some may have incorrect balance weight, pulley bolt patterns, and/or timing mark locations.
A lot of boneyards use the Hollander Interchange Manual to help determine which parts are interchangeable between which cars, trucks or engines. A Ford part's department should be able to tell you what years the correct torsional damper part number applies to, and new aftermarket dampers will be cataloged accordingly.