I've found 2 TC is best for dirt. Keeps the wheels under control at lower levels, and you can still let it lose traction when you want to. However dont drift every corner, the principle is opposite compared to gt4. Keep as much grip as you can, however there will be many corners where you need to swing it out and have some drifting fun
Go in your settings for the car scroll down to *Drivetrain* click to open it then go up to the box at the top which says something like "Torque Distribution Centre Differential" click on the tab which looks like a dial .. In there you can adjust to 50/50.
(sorry i am not at my PS3 at the minute if i have the wrong name for the device i appologise but it is something like that)