Buying a used transmission question?

I have a 1997 Chevrolet Blazer and this weekend and i buying a used transmission for my car. Originally it is 4wd. When i bought the car last year, the owner notified me that they "took out" the 4wd, Why? I have no idea! My question is, do i have to buy a 4wd transmission or 2wd? Because the car itself will not go into 4wd. Weird question, yes i know!