Short answer: Yes, the extension housing & shaft can be swapped.
Long answer: The shaft is installed from the front; so you have to pull ALL the internals out of the 2wd trans; insert the 4wd shaft, and reinstall all the internals minus the 2wd shaft.
At least that is how it was explained to me.
For me, if I have to pull the guts out, I might as well replace the worn ones in the 4wd transmission and have a transmission which I know will be good for years assuming the case was good. I just need to get over my fear of rebuilding an automatic.
If the transmission shop steered me incorrectly, hopefully someone will correct me. But the shop owner is a friend and knew at best he would sell me parts.