[David Fulford]

I'm not sure how fast you should slow down, there are several variables involved in that too, but if your rear end is making noise then it probably has inproper wear and needs some sort of attention. If your rear wheels spin opposite direction, it is an open differential. That is what I have always been told and what I've always went by. If I happen to be wrong, someone will correct me. I can't explane the physics of it but both tires do sometimes spin with open diffs and from my previous experience, usually when you get on it hard. Hope I've been of some help.