You have to find the oval opening in the backing plate that gives you access to the adjuster. While pulling on the adjuster lock bar (I make a hook from a coat hanger) you turn the adjuster wheel with a flat blade screw driver to back the brakes off. Sometimes the hole in the back plate will have a rubber plug, sometimes it will be marked but not completely punched and you have to punch it out. If you buy a spring kit for the rear brakes it should come with a rubber plug in case you have to punch it out.
This has nothing to do with rust. Over time, the shoes wear a groove into the drum braking surface, which creates a lip on the drum on either side of the shoe that makes it hard to get the drum off. I suppose you could beat it off (that didn't come out right) but the proper way is to back the adjuster off.
2001 Bonneville SSEi (retired at 365,000 Kms.)
2014 Jeep Grand Cherokee Overland (yes, I know its not a GM)