Early Land Rovers upto the series 3 were not permanent 4 wheel drive they had a dog gear which you had to engage to get the front wheels into mesh.
The later 90,110,130 Land Rovers and all Discoveries replaced the doggear arangement with a centre differential (Simimilar to the Range Rover of that time) making them permanent four wheel drive. Drive is permanently distributed to all four wheels. With all 3 diffs being open it is possible to get stuck if one of any of the wheels looses total traction (ie, its off the ground). To give these vehicles off road capability Land Rover fitted them with an electronicaly operated locking centre diff. This can be selected in high range and automaticaly engages in low range.
I drive a 110 every day and its possible in the wet when pulling out of junctions to get both the front and rear inside wheels spinning without difflock.
If we were to say the centre diff had to be locked all the time to be permanent four wheel drive then NO road car is permanent 4x4.
Permanent four wheel drive means that the drive is permanantly sent to all four wheels so the scoob and Disco's a have permanent four wheel drive. The fitting of viscous or electronic locking diffs are just there to further control traction.
Lee