I read your question and if I understand correctly you are wanting to put the ifs v6 rear axle housing into your 84 truck using the 4cyl 8" diff correct? If so, yes you can absolutely do that. Just build the 4cyl diff with 4cyl gears and drop it in the housing. Just dont try to put the 4cyl gears on the v6 3rd member and vice versa.