A buddy of mine also had the same problem after Ford replaced a snapped rear spring... Ever since ford touched the right rear to replace the spring it squeeks at random, usually low speeds, also a pre-98.
Thought of this when I read the very first post then I remembered the above and I'm pretty sure my buddy looked into his parking brake and it wasn't the source of the sound.
(Off the hip here.. Perhaps it's the parking brake cable rubbing, it makes a pretty annoying sound at random when it starts to rub the trailing arm.)
