I have a 90' Ford Ranger, rear wheel drive.
I can not find a 2 wheel drive automatic transmission and I was wondering if I can convert a 4 wheel drive transmission?

 

Welcome to FTE.

You would need the output shaft and tail housing--in other words, a donor transmission. Keep looking.....

 

I have the old transmission, will that work?

 

Swapping the output shaft requires disassembling the entire transmission. It's basically the last thing to be removed. If you're comfortable taking apart both transmissions to swap the output shaft (assuming your old one isn't damaged) then you can use yours.