Do most Explorers have disc brakes in the rear?

To set your mind at ease I think you'll find that while drum brakes may not be exposed to salt damage as discs are, there is a reason most newer cars have discs instead. Discs are far more reliable as well as effective for stopping plus they are easier to service. The hydraulic operating components of discs are completely enclosed meaning there's no mechanical parts such as springs, clips and cables. Anytime you can reduce the number of moving mechanical parts its a plus. As far as your Blazer's discs go... all brake set-ups, drum or disc, require routine maintenance including replacement of drums/rotors, pads/shoes, calipers/wheel cylinders as well as any of the mechanical components. That being said, any of the newer Explorers will have discs all around and there's a reason for it.

pretty much all cars do these days