OK, I finally did what I should have done to begin with, I dragged out the 10 lb. Ford shop manual and looked there. It identifies the 4x4 part as a "knuckle" and the 4x2 part as a "spindle". They're not interchangeable and they look different. As Doc said, the 4x4 part has a large hole for the axle half-shaft to pass thru whereas the 4x2 part has a large shaft for the wheel bearings, etc.
Thanks for the help.