An open diff will have opposite spinning wheels it the drive shaft is held in place. The posi-lock is basically a cable operated version of the vac motor. You can make your own if you wanted to make sure it would work prior to spending cash on a "pretty" unit. To me, it sounds like there may be a little wear on the collar causing it to "hang" when moving to the lock position. It should slide smoothly but not have lots of play.