tire size won't change anything as the size is the same in front and in the rear. What is important is the ratio size between front and rear. You could put 3" higher tires if you wanted, as long as you have the same size front and rear.
This is the first time i hear of someone who wants to change the rear diff, what is wrong with it? If the part numbers in the catalog are the same, to me they definatly are the same. As soon as they change a tiny little thing they change the part number so if they have the same number...