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The number of driven wheels You may not know this, but the average 4x4 is really only two wheel drive. Jack one rear wheel and one front wheel off the ground, and you will note that the wheels in the air are the ones to spin, not the ones on the ground. Even if your vehicle has a rear limited slip differential, the same scenario applies. So, what can be done to help in this situation? Well, this is the reason a lot of four wheel drivers fit diff locks. A diff lock fitted to just the rear diff, means that if you have three wheels in the air, the one rear wheel on the ground is enough to continue forward motion. The wheel in the air will spin at exactly the same rate as the one on the ground.