If you have a look at that link in my last post, it shows you the 1 wheel up test.
Nodge above refers to the IRD as Freelander's 'transfer box'. It is, but it is nothing like a 'normal' transfer box.
The IRD essentially just has a front diff (so the front wheels can rotate at different speeds) and a direct drive (PTO if you like) to the prop shaft. There is no 'center diff' - so you would get wind up in the transmission if the prop went straight to the rear diff - this would blow the transmission very very quickly. So the prop is split in 2 and there is a Viscous Coupling (VCU) separating them. When driving normally, the 2 parts of the prop are spinning at the same (or very close to same) speed, the VCU is 'loose' - so little drive is transmitted to the back axle. When the fronts lose traction and spin (eg ice, gravel sand), the 2 bits of the prop spin at different speeds, this 'thickens up' the silicon fluid in the VCU (to the point of it essentially locking up - thus providing drive to the back axle when you need it. This happens instantaneously, as does the loosening once the fronts get grip and the props are back to rotating at the same speed.
If you run mismatched tyres, or they are not all pumped to the same pressure, the axles (and therefore the 2 bits of the prop) will turn at different speeds and thus the VCU will stiffen (lock in extreme) all the time and create that harmful wind up. Similarly, the fluid in the VCU degrades over time (and poor tyres maintenance) so will become too stiff in its ambient state or lock up too soon (eg when cornering).
This wind up puts extreme stresses on all of the transmission, and it is usually the IRD that 'goes' first.
If you look after the tyres and their pressures and monitor the VCU (1WUT) to ensure it hasn't got to stiff, then all's fine.