The way I understand it is that the shaft necks down a little near the splines to absorb twist. I have aftermarkets shafts for several axles and they all have a little smaller diameter near the spline. When the shaft neck down after the splines, the twisting force is handled in the smaller area like a torsion bar and keeps the shock loads away from the splines. This makes the shaft stronger. I think Ford is just following suit. The minimum diameter of 1.356 occurs only in this small area or section of the shaft. I have seen the shafts from the Super Duty Dana 60 and they are really impressive pieces.

Joe