The older rear motor 4wd cars were necessary at the time for the rough tracks that most offroad racing was held on. With the smoother tracks that have been much more common the last 15 years, the 4wd has been utilized more for cornering power.
2wd needs the motor out back over the rear tires since it doesnt have the extra pull of the front tires. The kickup in the front helps to take away a little corner entry steering and really helps with bumps.