OK, my truck did not come with a posi. Yesterday I was at the beach up here in Trinidad, CA, where you can drive on my beach. I told my buddy not to take his 2x4 Nissan on the sand. While we were out there he asked what exaactly a posi is. So, I put it in 2x4, granny gear, to show him one tire spinning. Well, both tires went, not even for a second did one tire go and not the other. I did it again. Same thing. Out in this mushy sand, no matter what, both rear tires spun. I don't want to pop the cover over 600 miles away from home. Would this ever happen with an open rear?

 

In an open diff both axles always get equal torque but can turn at different speeds. So it's not unreasonable to see both wheels spinning. If they do it only means they have equal traction at that moment. Now try putting one wheel on ice and one on pavement. Clearly, only one will spin. They are getting equal torque, but one has more traction.

John