I have 27,000 miles on Ram dually and the rear tires are worn down to less than half tread left fronts are fine is this normal with dually's. I always keep tire inflated to the reccomended pressure.
Don't feel bad. My 2500 does the same. I get them rotated to even them out. To many rt. turns in the cities trying to get into traffic and no weight. Even my 1500 5.2L powered did the same thing. I average 55k out of a set on my 2500.