I now think I know what the problem is.
I'm pretty sure it's because of a piece of metal rattling around in my driver's side rear wheel.
I put my rear end up on jacks, and spun the wheel around, and I could hear something rolling around in the wheel.
Also, there is some sort of large metal screw embedded in the wheel. I'm thinking that a piece of the screw probably broke off inside the wheel.
Also, the driver's side rear wheel always has the least air pressure. I just checked all of the tires, and here are the air pressures:
Front Driver's Tire = 40 psi
Front Passenger's Tire = 40 psi
Rear Driver's Tire = 18 psi
Rear Passenger's Tire = 28 psi
They were originally all about 34 to 40 psi.
This situation has existed for about 6 months now. I'm afraid to remove the metal object, because maybe it will make the leak worse.
All of my tires are original, and have about 36,000 miles on them, so maybe I'll just get new rims and tires. I'm thinking 17x10.5 in rear, 17x9 in front. Probably chrome Bullitts, or maybe those solid rims that have only a few small holes in them, like you see on a lot of old muscle cars. That may look good with my 1995 Cobra R hood and deleted spoiler.