I've done a lot of reading up on viscous couplings recently:
VW used them to drive the front wheels of their "synchro" vans. They have many of the same issues with tyres etc as Land Rover Freelander VCUs. VW Viscous couplings can leak (not a problem for LR VCUs) causing no drive to front wheels, or gellation of the fluid causing constant drive (as with LR).
From: http://www.syncro.org/VCTest.html
Also, I would wager the below (form the link above) is the primary reason for LR Freelander VCU failure, especially as the early vehicles gearing meant the unit was constantly slipping and hence heating the fluid:
See also: http://www.vanagon.com/syncros/technica/ for some more info.
It would appear Land Rover VCUs (like VW) suffer from "Gellation" where the fluid spoils and goes thick, causing transmission wind up (perhaps due to the unit constantly accounting for the mis match in gearing). So even if a car passes the "VCU test" of jacking up and turning a rear wheel, the fluid may still have spoilt to a certain degree and be causing a load up of the Diff and IRD - i.e. its not totally solid but is dragging.
I suppose the test should be changed to "if the rear wheel is hard to turn" - you would need to compare a new VCU to yours to tell - if yours is harder to turn then the new VCU it might be on its way out. But I would like to see other evidence like wear patterns on tyres and the reverse skipping tyre test. I would say going by the expense of new IRD and diffs then prevention by fitting a new VCU to a car that has the reversing issue and scrubbing rear tyres would be wise.
This does negate the common belief that if a Freelander is used off road a lot then the VCU will be healthier - unless of course regular "cycling" of the fluid helps keep it in good condition, but I do not know enough of the chemical composition and properties of the fluid to comment on this.
This link indicates it is possible to refurbish a VCU: http://www.vanagon.com/syncros/technica/vc-expert-interview/index.html You can imagine drilling out the fill points on the VCU, blowing the fluid out with an airline, fitting brake nipples or similar and refilling with fluid. However it seems the sticking point would be locating a supplier of the correct fluid! We have no idea of the exact composition of the fluid Land Rover used.
These guys refurb them and sell for £200 - they must get their fluid from somewhere - but £200 seems pretty reasonable if they have a warranty:
http://www.bellengineering.co.uk/5.html
Hope this info helps people understand the VCU!