I know that this is an old thread, but the principles are useful, and nicely explained here. I thought I would try to add something, both theory and practice. Someone chip in if I get anything wrong. If you are going in a perfectly straight line then differentials don't matter. Once you turn a corner then they do. Imagine turning in a tight circle on a muddy surface, so you can see the wheel tracks. The outer wheels have to travel further that the wheels closer to the centre of the circle in the same time, so obviously need to travel faster than the inner wheels. To allow this, there is a differential on each axle to allow each wheel on the axle to travel at a different speed, yet both be powered by the engine. Now think of the rear wheels versus the front wheels. Because the rear wheels don't steer, they don't follow in the tracks of the front wheels, but 'cut the corner' so to speak. So the rear wheels travel more slowly when going around a corner than the front wheels do. The front inner and rear outer are not too different (depending on wheelbase) but there is a big difference between the front outer and the rear inner. For a normal car this doesn't matter, because only one axle is driven anyway, but LandCruisers have full-time 4x4, so there is a differential between the two axles as well. This is why you don't engage the diff locks when on a solid surface, the wheels need to go at different speeds but you won't let them, which puts a lot of strain on the axles, and will also force some of the wheels to lose traction. If you find you can't disengage the diff lock after using it on soft ground (often because you came onto hard ground with the steering wheel not straight), it means there is torque holding the diff lock in place, and not letting it drop out. So you straighten the wheel and go backwards and forwards a few dozen yards to release the torque. Marlot mentioned that you engage centre diff-lock (HL, or LL for lower ratio gears) if one of the wheels loses traction, and rear diff as well if you lose traction with a wheel on each axle. In a non 4x4, one axle doesn't matter, you just have to lose traction on the driven axle, but in a 4x4 they are all driven, so losing traction on any wheel is enough. So why have full time 4x4 (the Mitsubishi Shogun doesn't, eg)? In order to move the weight of your vehicle forward, you need to apply a force to the ground through the tyres. If you are powering only two wheels, the entire force is split between those two, but with 4x4 you spread the force over four wheels. So although 4x4 means you have to engage diff lock if any wheel loses traction, you are less likely to lose traction in the first place, because each wheel requires less traction to move you forward. Also, whether off or on-road, it is still true that if you turn the vehicle the wheels need to travel at different rates, and if you lock the differential you stop that, so forcing one or more wheels to lose traction. Therefore, even off-road, you don't engage diff lock UNLESS you have lost traction, and only until you get to ground where you have traction again. Finally, a practical example. I live at the top of a steep winding track in scotland. During winter, that track is covered in an inch of ice with snow on top. The LC has no problem with this. At the top, I go into LL 1st gear and lock rear diff as well. Then point downhill, and keep my feet off the pedals! In low range first it goes down with engine braking at walking pace, and with all diffs locked, one of the wheels losing traction doesn't mean I suddenly lurch forward. As soon as I'm at the bottom, disengage all locks, go into high range, and drive off. Works like a charm.