Its not user selectable - its an automatic kind -http://www.youtube.com/watch?v=kL0hPEOStGk
Dude.....after 3 years of watching my friends -who own 1000$ suzuki vitaras with a simple rear diff lock do stuff that i could only dream about - i took the first diff lock i could find!!!!!!! dont care if its an automatic type - an air pressure type or a chicked poo operated type - as long as it works!!!!! and mine works.
BTW - automatic lockers are more reliable - pressure operated ones can fail on you when you least expect it.
here is a quote from wikipedia about my kind of locker -
Automatic lockers lock and unlock automatically with no direct input from the driver. Some automatic locking differential designs ensure that engine power is always transmitted to both wheels, regardless of traction conditions, and will "unlock" only when one wheel is required to spin faster than the other during cornering. They will never allow either wheel to spin slower than the differential carrier or axle as a whole, but will permit a wheel to be over-driven faster than the carrier speed. The most common example of this type would be the famous "Detroit Locker," also known as the "Detroit No-Spin," which replaces the entire differential carrier assembly. Others, sometimes referred to as "lunchbox lockers," employ the stock differential carrier and replace only the internal spider gears and shafts with interlocking plates. Both types of automatic lockers will allow for a degree of differential wheel speed while turning corners in conditions of equal traction, but will otherwise lock both axle shafts together when traction conditions demand it.
Pros: Automatic action, no driver interaction necessary, no stopping for (dis-) engagement necessary
Cons: Increased tire wear and noticeable impact on driving behavior. During cornering, the automatic locker is characterized by heavy understeer which transitions instantly to power oversteer when traction is exceeded.
Some other automatic lockers operate as an open differential until wheelspin is encountered and then they lockup. This style generally uses an internal governor to sense a difference in wheel speeds. An example of this would be GM's "Gov-Lok."
Some other automatic lockers operate as an open differential until high torque is applied and then they lockup. This style generally uses internal gears systems with very high friction. An example of this would be ZF "sliding pins and cams" available for use in early VWs. [color:"red"] [/color]