Losi SmartDiffs are not true torsens, but they are available.
From time to time they get a bit of wordplay, but in the end, they aren't worth it.
http://www.losi.com/Products/Default...rodID=LOSA3603
At one time, Fioroni made a true torsen (Fioroni Tractive IV Differential). Don't know if they are still available.