Like all road cars the LR has differentials across each pair of driven wheels to allow the outer wheel to travel faster around a corner than the inner wheel.
When you have 2 driven axles they also travel around a corner at different speeds - the front axle has a greater distance to cover than the back axle.
So, a third differential is added between the 2 axles to allow each one to turn at a different relative speed to each other. So yes all 4 wheels have power available to them all the time, however as the guys above have said when off roading the power takes the route of least resistance so what can happen is if one wheel on one axle is waggling in the air, it will steal the torque from the other wheels on the ground, spin, and the car stops moving forward.
What your difflock does is force the front and rear axles to turn at the same speed, which will mean you keep moving in the above scenario because there is 50% torque sent to front and back axles. So instead of a minimum of 1 wheel drive with no difflock, you will power at least 2 wheels with the lock engaged.
A cross axle situation where diagonally opposite wheels are off the ground will out smart the difflock. What you need then is either traction control or a second locking diff in an axle. With that engaged you will get 50% torque to each axle, then on the axle with the second locker both wheels will be forced to turn together. This guarantees at least 3wd.
A locker in the second axle will result in true 4wd, where each wheel receives 25% of the total torque.
Hope that helps, difficult to explain without knowing how much you understand mechanically but please ask for more if you need!
