Time to conclude this little sqeeling noise.
I had time to check my rear brake pads again. Took off the outside pad on the left rear of the car and while the pad material is low, its still useable. In fact, I noticed that there is a metal wear indicator on the pad and the pad material was still a couple of millimeters above the wear indicator. When I checked the inside pad, I noticed that the pad was worn down to the metal indicator tab and the tab was freshly worn. However, the pad material on this inside pad was as thick as the outside pad, which was still 2mm away from the wear indicator. So I simply pushed the metal wear indicator tab on the inside pad back away from the pad surface by approx 2mm to even it up with the outside pad. Reinstalled pads and test drove the car - no noises.
Summary - if you have a constant squeeling noise on acceleration / desceleration, turning left or right, then check your pads as one or more are likely worn down to the metal wear indicator tabs, which cause a horribly loud squeeling noise to notify you to change pads.
BTW - does anyone know what the thickness of a new rotor for a 91 LS 400 is supposed to be (front and rear)? Also, what thickness on a worn rotor would be considered out of spec and necesitate replacement rotors?