I chose to ressurect this thread as I navigate my quest for "differential knowledge". My low-performance brain is far too under powered to decypher all of this insightful data. I'm just curious as to what lurks in my undercarrage (no puns please). During a recent wheel-balancing episode, I noticed the techs lowered one of the rear wheels onto a rim to hold it still while they spun the other using my cars drive train. Then they swapped ends and spun the other. They referred to the differential as being "open". I'm pretty sure that both wheels spin when laying drag.

So what it be?