I just replaced the stock Continentals on mine (after 53000 miles) with BF Goodrich Rugged Trail tires. Good price and should get just as good of mileage out of them. I priced these tires at several tire stores and they were $1300 and up.
I did price the Toyo Open Country tires and they were going to run $1800 or so.
Finally I called my Ford dealership and asked the service advisor what other tires Ford dually's come with and he suggested the BF Goodrich rugged trails. He quoted me $990 for all 6. I got all six, mounted balanced and out the door for $1050!!!