93 Ford Explorer Starter won't disengage from Fly Wheel

Recently my 1993 Ford Explorer (automatic, v6, 4wd, XLT) would not start. I went down the list of possible culprits, checked the battery, cleaned the wires and terminals and posts, replaced the starter solenoid relay next to the battery, and then finally the starter. Once I removed the original starter, I had it tested, and it was definitely bad.

I replaced my original starter with a used starter from a salvage yard, and the car turns over and starts, but the starter would not disengage from the flywheel. It makes the same sound as a running car that gets the key cranked. The engine and starter both stop when I turn the key to the off position.

I thought the used starter might be bad, so I bought a brand new starter from the auto store, installed it, and got the same problem. The starter wouldn't disengage from the flywheel.

I did a bit of research, and it was suggested I replace the Ignition Relay Switch in the dash, above the steering column, so I did that, and have the same problem.

I read that some cars require a shim to be placed between the starter and the transmission bell housing, but I was told that my model Ford doesn't require a shim.

I'm not really a car guy, and I'm out of ideas. As far as I know, I've installed everything correctly. Any advice would be greatly appreciated.

Thanks in advance,
Nick from OK