In 4X4s with permanent 4 wheel drive, i.e no two wheel drive option, there is a central differential as well. You have to have all differentials free/ unlocked to allow for various rotations on all four tyres while turning as if you have locked shafts you will not be able to turn as it with damage your drive line seriously. The diffs can be locked to help improve traction. With all diffs unlocked on slippery surfaces, even with one wheel spinning your vehicle may come to a halt, because all torque is wasted on that wheel. With centre diff locked, usually with a switch control or lever in all proper off-roaders (soft roaders may not have that option), the torque supplied to front and rear is same/ equal, so both drive shafts (rear shaft and drive belts in 1st gen pajero) would be rotating at same speed. Now if you have traction than thats 4 wheel drive but on slippery surface or articulation scenarios where one front and one rear wheel loses traction or is in air, you will be stuck and in that scenario you essentially only have two wheel drive. To overcome that scenario there are individual rear and front diff locks. In all part time 4 wheel drive vehicles, when you change from 2 wheel drive to 4 wheel drive you get locked front and rear drive shafts spinning at same speed, so it acts as central diff lock.
Your second question is actually pointing to locking front wheel hubs rather locking diffs if I am not wrong. Wheel hub locks are used to disconnect wheels from the drive shaft to help fuel economy and general wear n tear.
Usually vehicles would have centre diff lock first, some would have additional rear diff lock only and finally some would have all three together, you would not see front lockers alone, unless someone has put in aftermarket ones, but these too will only be along centre diff locks.
Link to a recent forum on this topic