If you had both rear wheels locked, that would put a lot of undue stress on the CV's. That is why they are generally left as an open diff. The fact that the free wheel spun while the other was stopped, further reinforces the idea that it is indeed, an open differential. From the factory, spools and open diffs are simple, and cheap to mass produce, but spools break things, just ask anyone who had run one. The LSD is complicated, and expensive, and somewhat failure prone when mass produced in a low/no QC environment. I will link an old tube vid on diffs that I watched when starting out in my field, and while it is ancient, it effectively explains differential need and operation.
http://www.youtube.com/watch?v=yYAw79386WI
Here is a Toyota instructional on types of diffs as well.
http://www.youtube.com/watch?v=gIGvhvOhLHU