The car had when you purchased it

205 sierra cosworth engine
Full 2wd sierra cosworth running gear including rear beam and diff.
The front end is still 4x4 minus the diff and drive shafts. I believe some kind of blanking plate was inserted into the hubs which are 4x4 on the front.

If your going for a bigger dif. Look into the supra diffs. Cheap and handle massive power, they have made a fitting kit to the 2wd rear beam so bolts straight into what you have.