Assuming you are talking about the axle shaft, you are contemplating saving the flanged end and the splined end and taking 4 inches out of the middle, then welding it back together. I have seen a few axle shafts that twisted and broke, these were medium duty trucks. The shafts appeared to be forged steel, with a grain that was pretty apparent. Without knowing the metallurgy of the material, I think it is weldable but I doubt you will ever achieve the original strength. If you have several shafts to experiment with and little cost, might as well give it a try.
