You dont have to replace the 21 spline gears to do a dual setup, its just the coupler between the 2 that you would need to have either 21 or 23 spline.  If I was gonna do this swap Id do 4.7's in the rear case that way you'd not only have a stronger input on the rear case and crawler gears to boot but youd actually be close to the same money considering you would need to buy a new input gear anyway