This may be easier to understand. Transmission output 21--> crawlbox input 21...crawlbox out 23--> rear case input 23. Keep in mind that when you buy a 23 spline adapter yu are only adapting the crawlbox output and not the crawlbox input where it couples to the transmission.
duh sorry i had a brain fart but yes, i get it now i will order all that, and how much stronger is 23 then 21?