Only used to driving Landie 110s for 5 years so I may not be able to answer but - the 110 is permanent 4 wheel drive with a seperate stick to select low range gears i.e. the gears you use when trying to get out of mud, snow, ice etc. it's always 4wheel drive but virtually has '2 sets of gears'.
I believe the Disco's are pretty much similar. Range Rovers, Land Rover Freelanders, Discos and the 90 and 110s share a lot in common for chugging off road, even if the shells are built completely differently.