what 6x4 could mean

In a 6x4 all 4 driven wheels are always working.The power train goes via the gearbox to the prop shaft to the differential on the front leading axle, and is then transferred to the rear axle differential by an inter differential prop shaft.They operate as any normal truck would with both differentials operating in tandem.
The purpose of a differential lock is basically to lock both drive shafts LH and RH to rotate at the same speed(i.e they no longer have freedom to rotate at different speeds around a curve).It is generally operated by a pneumatic switch in the cab and is used sparingly in cases where wheel slip can occur so as to provide equal traction to both wheels.The lock is disengaged once the truck is out of the rut.It is not advisable to run with the differential lock on for extended period of time as it will stress the drive shafts particularly on curves.

 

This is wrong, all 4 wheels have power going to them at all times, that is why if one of your front drives looses traction you won't be able to move if you don't have the power divider in because all of the power will be sent to the wheel of least resistance and since it has no traction it will just spin and you won't go anywhere. The reason your rear most drives wear quicker is because of the way a truck turns it actually pivots on the front wheels and the rear wheels scrub because they take a wider arch. I use this to my advantage everyday getting in and out of tight job site. I know that if my front drives miss and object as I am turning around it the back drives will miss also