Far as I know the only time it isnt used is when going to 4low. I know mine has gotten stuck in part time when going back to 2wd from full time.
If you want, you can lift one of the front wheels and play around with it. Its really easy to hear and feel when it goes in and out of 4wd. Shift it to part time then try to turn the tire. You should hear it pop right in(given its quiet out). Do the same going into and out of full time. Its pretty neat, I enjoyed it, haha.
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LOST KJ'S
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Current Jeep: 2004 Jeep Liberty
-Atlantic Blue
-Air Ride Rear Suspension ; OME/Rustys Lift ; Powertrax Locker ; Moog Lower Ball-Joints ; 245/70/16 Yokohama Geolander A/T-S's on MOABS ; Flowmaster 50 SUV
Lost #007082
http://www.cardomain.com/id/jeepjeepster