Decoupler

From VanagonWiki

A decoupler is a mechanical device (operated by the same type of vacuum powered actuator as the Syncro's rear diff lock) which disconnects the front power output shaft of the transmission from the driveshaft (and thereby also from the front differential, front axles, and wheels), effectively turning your van from full time AWD (via the viscous coupling) into a 2WD van.  The decoupler replaces the front nose cone on your Syncro transmission. 

A decoupler can be purchased from several vendors with slightly different variations in how they achieve the decoupling effect.  The first decoupler was one originally produced and available on some German Syncros.  In the US, Tom Lengyl designed his type which is now sold by most of the popular Vanagon vendors.  Kirk Meeks also sells a kit of parts that you can convert your existing front nose cone to a decoupler type.

There are many, many discussions about the pros and cons of the decoupler with passionate debate both pro and con.  I will not contribute to that debate in this post.