I just went to look to look at a 4Runner today and overall the undercarriage is clean for being in the north east. Surface rust here and there..
The only troublesome area (maybe.. I've never dealt with drums) is the drivers side rear backing plate. As you can see in the photo below (sorry for the blur effect) it has started to rot.
What do you think? Is it nothing to worry about or is it a high priority fix? I'm leaning towards the latter: