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.
I am also surprised that the rears wore faster than the fronts. My truck has about 33,000 miles, I rotated the tires somewhere around 20k. The tires that were on the front (now outside rears) are almost down to the wear bars.
I've had diesel 2500/3500s since about 1999. I don't think I have ever been able to get much more than 40,000 miles out of a set of tires. Others claim 100k, but with that heavy of a truck that is used for towing those numbers seem unlikely to me.