brake noise when backing up

I can hear a rubbing noise when backing up without the brakes applied (parking brake off) with my 2000 GLS. The noise is a short moan-like squeak that repeats several times depending upon how far I back up. I was wondering if it might not be the parking brake shoes contacting the drums. Is there an adjustment I should make?

Are you sure it's not worn pads. Most have a wear indicator that can make a similar noise. It's not uncommon to first hear it in reverse.

I just had the tires rotated and balanced, and while they were at it, I looked at the pads, they are about 75-80%.