Gentlemen, hold your horses please.
You have got it entirely wrong.
In fact there are two (2) differentials in every 4 wheel drive vehicle. One is in the front axle and the second is in the rear axle.
This feature distinguishes 4wd vehicles from 2wd vehicles. 2wd vehicles have differential gear either in front if it is a front wheel drive or in rear if it is a rear wheel drive. There is no second differential.
In normal, 4wd vehicles in 2wd mode only the rear axle is driving the vehicle. When you engage front axle through your transfer gear, both the front and the rear axles are locked together and engine torque is transmitted 50/50 to both front and rear axles. So in these part time 4wd vehicles, at least two wheels, one in front and one in back should spin simultaneously to stop the vehicle from moving.
The gentleman @chall3ng3r is confusing all the above with a third differential, which is installed in some 4wd vehicles along with the transfer gear. These 4wd vehicles have a special name which is "full time 4wd". These vehicles like previous gen Fortuner or current gen Prado has following configuration on their transfer gear lever i.e., 4H, 4Hc, and 4Lc. There is no 2H in these vehicles..
For these particular vehicles, if only one wheel starts spinning or slipping either on front axle or rear axle, they stop moving. This is the reason these vehicles always have a "centre differential lock" which is engaged when you shift the transfer gear lever in 4Hc or 4Lc. In these gears, these vehicles 4 wheel drive system works exactly like the normal 4wd vehicles i.e., you have to spin two wheels, one in front and second in rear, to stop the progress.
Additionally, differential locks can be installed on rear or even on front differentials. Current gen Fortuner/Revo has differential lock on rear axle. These vehicles need to spin both rear wheels and one front wheel to stop their progress.
LC 70 newer models have also front differential lock and these stop only when all their four wheels slip or spin. they keep on moving even when only one wheel has traction/grip.
Hope the above helps and clarifies some of the confusion.