Davidka, that's exactly right, if the centre diff is locked the car will want to lift the front end off the ground and you will have zero control down the straight. With some centre diff action the car will stay flat under acceleration and maintain control.
Another setting that I've found that makes a HUGE difference is droop settings. Less droop on the front on smooth tracks, the cars front end can only lift so high on acceleration keeping the car flat and allowing more power to the ground. If the track is bumpy then add some droop and it will take the bumps better. On the rear less droop will allow the car to pivot (drift) in the corners under braking (won't unload to the front as much), with more droop the car will brake harder (loads up the front wheels more, better for loose/sandy tracks).
Also, always set your brakes with more rear bias (i.e.: more rear brakes than front). This will act like a one way, the front wheels will keep turning under braking allowing control, and the rears will lock up allowing the car to pivot around corners quickly (drifting).
I've found that the fastest way to get around the track is to ALWAYS keep the car in motion, master the art of drifting the car around the corners and you will see your lap times go down significantly.