Its rather complicated, but lets give it a go;
Diff locks in 4X4s are: Central, to make sure equal torque distribution to front and rear wheels. Additonally in some vehicles there are separate front and rear diff locks to make sure that equal torque is sent to right and left wheels. Theoratically one would have the most traction with all diff locks engaged, however lots of other things come into play (to avoid wheel spinning/ improving traction) including the tyres and suspension (tyre contact/grip to any surface depends on your suspension, be it off-road or onroad vehicle).
Limitations of full diff locks is that it can be used on only slippery surfaces as wheel speed on all 4 tyres is same. Which mean that transmission is under lots of stress when turning etc because wheels are covering different distances in turns (ouside more that inside tyres). You can actually snap your transmission on grippy surface by constantly turning with full diff locks engaged.
Diff locks are meant to help in avoiding the wheel spins, so you can keep moving forwards (or backwards). Theoretically in most 4X4s, when on steep uphill slope with two wheels on ice/very slippery surface, the vehicle would not move forward, but rather the wheels on slippery surface would keep spinning. So in real term these vehicles are not 4 wheel drive. However if you have full locks on than this could be avoided, as tyres on left and right side can only turn at one speed, so no power is wasted on spinning.
Manufacturers use various other things to avoid wheel spin as well, like Limited slip differential, torsen diff locks, traction control systems etc.
Most Proper 4X4s would have central diff lock. Soft roaders mostly wouldn't. Very few 4X4s would offer full diff locks even as an option. G-Wagens have offered them standard from beginning (apart for G500 for some model years), Some models of Jeeps and I beleive Hummer H1 also had them.
As far as I know Defenders and even other land rovers/ range rovers would mostly just have central diff locks. But they mostly can make up for that from there excellent suspension.
Landcruisers series 80 had front and rear diff locks only as an option. In Series 100 only rear diff locks were available. Not sure about series 200. I think its similar for old and new Prados.
As mentioned in other replies, you can also get some after market diff locks, including full diff locks.