Like the others have said, the load rating is definitely not the issue. If you are wearing out tires in 20,000 miles there is something else going on, assuming you are using quality tires. Even on a heavily loaded one ton you should be getting 50,000 miles out of the front tires quite easily.
Agree with this, and the other suggestions of what could be going on. 20k is far too short for front tires on a dually that is in good shape.
I've owned a few, my dad has had duallies since 1986, and my FIL has owned a couple different ones. NONE of them have gone through tires that quickly, with the exception of the time my dad got a set of Michilens on one that happened to have defects (this was probably 20 years ago now). He blew 3 tires on a trip back from out west - the tire company inspected the carcasses (because he saved them - he's like that), and ended up paying for all the repairs AND a new set of tires.
You may have an alignment issue, worn/damaged suspension components, or it could even be your driving style. If you're running loaded and still taking turns hard, you're adding more force to the tires than they should be seeing and wearing them out.
I'd start with an alignment, but remember that not all shops can properly do an alignment on a dually truck. Some of them just don't have an alignment rack big enough to do it. You may have to look around to find one.