You can still convert it - but you'll need a new mainshaft (from a 4WD version,) and a transfer case adapter housing (from a 4WD version.)
Dig around and see if you can score a "builder" or a "core" - and cross parts to make one good 4WD transmission.
But, yes, you should have asked. I'd have told you much the same thing I'm telling you now - but $300 earlier. And that's if I'd answered first - someone else, with much the same information, would probably have beaten me to it.
5-90