It's a locking rear diff. The ultimate traction device. It keeps power to the opposite wheel when a wheel on the same axle loses traction. As a factory installed option it is built into the rear end.

If you add it later not only will you have to purcha$e it but you will have to pay someone



to have it installed.
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If we do not succeed, then we run the risk of failure. - Vice President James Danforth "Dan" Quayle