Sure Grip or not

I believe backing up is an old urban legend. at least for Mopar. I heard that too when I was a teen in the early 70s.
Another way to check,
With only one rear wheel off the ground and trans in Nuetral. If you can spin the wheel its open, cant spin , sure grip.
With both rear off ground and trans in park (or in gear for manual) if you can't spin, sure grip, if you can spin the other one better be going the other direction or you likely have a broke axle.