what he means by step off is that some people (like me) need to run a lower ratio rear diff for acceleration but require the strength of a std cossie front diff. Meaning there is a difference in diff ratios.......The gearbox can accomodate this by using a step off gear which corrects the drive ratios internally so all 4 wheels drive as they should. A consequence of having disssimilar ratios is VC failure.

If you are running std diffs get hold of a spare set (you will need them for when your 600bhp chews the front in minutes) and swap themand see if the VC fails again.

Cheers
Dan