Differencial slip posted by Coolspot on December 02, 2014 at 08:12 PM
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They will only slip in relation to one another when enough force is applied, like when the vehicle is turning a corner. The outside wheel must travel farther, and faster, than the inside wheel.
If they were indeed locked together this could not happen. You should not be able to make it slip on the bench, by hand.
That is also why true locked differentials will not work safely on the street.