metal on metal noise - 4x4 s-10 Blazer
 

I have a '98 4x4 s10 Blazer. It has 4 disc brakes. Recently there is a scraping sound, definitely metal on metal, coming from the rear (I think driver's side). My first guess is a brake shoe is sticking but it has gotten louder and louder over the past week. I'm wondering if it could be wheel bearings, but I don't know what that sounds like. The noise I'm hearing is a steady scrape and it doesn't seem to change when I press the brake pedal. I've always done my own brake jobs and it has been a few years since I change out the rear shoes but they wear out so much slower than the front shoes. I'd like to hear your opinion.
Thanks in advance.