Very simply - the Freelander is basically a front wheel drive car that goes into 4wd automatically when the front wheels slip.  Drive to the rear is basically a power take off from the front diff and it does not have a traditional transfer case.  The front diff and rear pto unit is the IRD.
Most awd vehicles have differences in the front and rear diff ratios to enhance drivability and to activate 4wd quickly (this is done via the viscous coupling).  
In early Freelanders this difference was too great and caused the VC to work overtime and after some time (100,000 - 130,000km?) the VC would fail and lock up so the Freelander was now contant 4wd with no way to relieve transmission windup - the load causes the PTO section of the IRD to fail - bearings give way making the crown and pinion in the PTO section  to loose mesh.  If nothing actually breaks and it is a bearing failure (ie nothing fails so the front diff is still intact) you may be able to drive after an IRD oil change and removing the rear drive shaft and VC.  There  is no load on the PTO and the damaged bearings are often able to cope with this.
The system does not fail overnight and if you regularly test the VC to detect its failure the rest of the system can be protected.
Later and replacement IRDs have a better front diff ratio and there is no problem.  The above is a simplified version - but basically describes  the system.
However - what would be a greater concern for me is the 1.8 petrol engine - they are a grenade and will fail  - sooner or later - there is a fix but if it were me I just would not risk it.  The IRD may have already failed and be fixed but even if it hasn't it can be monitored to detect failure before damage.
If it is a 1.8 petrol I would stay away.
There are plenty of threads on here about all the issues so have a look at those.  Also on the IRD issue Ashcroft Transmissions
There are fixes for the 1.8 engine and the fix for the IRD is to instal a new one or one out of a post 2000 model.
Garry
				
			 
			
		 
			
				
			
			
				REMLR 243
2007 Range Rover Sport TDV6
1977 FC 101 
1976 Jaguar XJ12C
1973 Haflinger AP700
1971 Jaguar V12 E-Type Series 3 Roadster
1957 Series 1 88"
1957 Series 1 88" Station Wagon
			
			
		 
	
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