Peter,
a trutrac is a stronger version of a LSD, it is made by detroit/eaton but rather than friction plates it uses helical gears to force drive from the spining wheel to the wheel on the other side of the vehicle that is not slipping. It is stronger than your standard diff, cheaper than a manual locker and more flexible than a detroit or Lokka style diff lock.
If you follow these links you will see a picture and some more info.
http://www.aulro.com/afvb/introducti...trac-info.html
Truetrac helical gear limited slip differential
Their are a couple of distributors on Aus but it's pretty dear to buy one here. There have been a couple of bulk orders organised by other members from this forum on a couple of occasions in the last 6 months. But it was inconvienient for me so I made some private enquiries to the US as mentioned in one of the links above and got one sent over from US.
Due to dollar falling not as beneficial at the moment but still a lot cheaper than buying in Aus.
Hope this helps
Regards
Barjop
98 Tdi Auto
2" Ridepro lift
Rear trutrac