I think the splines are the same, but the axle is larger in diameter. There may also be a length difference, and I do know that the truck has different length axles on each side.
Are you saying that while the splines would be the same, the smaller diameter
5 lug axle would not work because of the difference in bearing size?
The parts catalogs do not specify a difference between 5 or 6 lug
drums, shoes, wheel cylinders, or front and rear wheel bearings.
Info from Classic Parts and Jim Carter.
Jerry