FS/FS2 is awesome!!
Its an odd car though in as much as the difference with and without the one-way diff is so limited people have driven mine and guessed wrongly if I have one fitted or not!!!
I always ran with the one-way as I like the disconnection of the front end under braking and lift off and thus struggled with perm 4wd. I have just started training myself to drive without one and it is hard work but is getting better every meeting (as in the driver is letting the car down less each time).
The option is there mate, the car is awesome either way ... MUCH smoother than a B44 without loosing out on steering.
I have a few bits in development too that might well help those people, like me, who prefer a one-way but would rather have diff-like front end drive ... like a centre one-way but better
