it comes under the rather dubious Central Rotational and Permanent drive system, commonly referred to, on here, as CRAP.
basically the IRD splits the output from the gearbox so that yu get a drive running towards the back via a propshaft - this goes thru a VCU or "Two tin cans filled with jelly", to give it its full technical description. This VCU is then connected to the rear diff (complete with mounts if yu are lucky) via another propshaft.
Wot happens is, when one of the rear wheels starts to slip on muddy ground or a wet leaf, the VCU says "bugger this" and locks up solid to provide drive to the rear as well. Unfortunately any slight tyre wear on the rear tyres and this confuses the CRAP system and the VCU decides to remain locked up. THis then causes the the IRD to go into hyperbole and screw itself
Hope this helps
love Dorothy