2wd trans to 4wd trans Q's

i have a 4x4 parts jeep in my driveway and it ran and the only issue was the trans didnt go into reverse. was told its a control valve or something of the sorts. now i'm about to pick up a clean 2wd XJ to convert to 4x4 and have a one good 4x4 jeep. i know you can make a 4x4 trans out of a 2wd trans by swapping the tail housing, shaft, and some other thing. so in theory the 2wd control valve would work on my 4x4 trans? and if that wasn't the issue i could build a 4x4 trans out of the good 2wd one, correct? i just want to make sure of my plans before spending money,


thanks
-ian

You can, but it pretty much involves a full transmission rebuild, since the whole gear train is stacked into the housing from the front. Everything has to come out, then you swap the output shaft, then everything goes back in (with new gaskets/seals/o-rings etc.) At that point you might as well just rebuild the 4x4 one and install it, keeping the 2wd one as a known good spare / parts box.

What year is the 4x4 donor and what year is the 2wd recipient?