More VCU...

Can someone explain why it is possibly 'catastrophic' when the VCU packs in? How exactly does it bugger up the rest of the drive train?

I ask as I also have a Discovery 300tdi which is in permenant 4wd, so I'm trying to understand what the difference is between a permenantly engaged (worn out?) VCU and the Tdi 300 system?

Any offers?

The Hippo drives the front wheels faster than the rear wheels. The VCU slips in order to compensate for this. If the VCU fails, the prop will put stress on the IRD and cause it to fail also.

if the vcu becomes too stiff it would be like driving with difflocks on all the time.
70mph in 4wd on the motorway would seriously wind up your drivetrain and break the final drive and the ird.