A freelander does not have a centre diff. It has a long propshaft from the gearbox to the rear diff. In the middle of this is the viscos coupling unit (vcu) which compensates for the different rates of axle rotation.
However, this VCU doesn't work very well and eventually you will get transmission wind up leading to failure of the gearbox or rear diff. To stop this happening, remove the propshaft, leaving it in 2 wd.
If you require a real off-roader buy a discovery.