Or...
buy a donar with the gear ratio you want in front and rear diff's and change them both.If you watch and not in a rush you may get lucky and end up with cheap diffs and some stuff to sell to cover the costs.
90 Chev 3500 c/c 4x4,6.2na,400 auto,4:10 gears.DSG Timing gears,main girdle, isspro tach, pyro,boost,oil and trany temp.Dual Tstats, High volume peninsular pump,on shelf, Custom turbo and intercooler 85%complete. Change of plans for the dually, it's going to get a Cummins. Both trucks are Blue 90 4x4 crews