From my experience, this is how the diff settings affect the cars in ISI games.
FWD:
More power = more diff lock under throttle which gives a bit more understeer
More coast = more diff lock while braking which also gives a bit more understeer
More preload = more understeer while maintaining speed. Doesn't seem to affect things that much though unlike the other 2.
RWD:
More power = more diff lock under throttle which gives a bit more oversteer
More coast = more diff lock while braking which also gives a bit more understeer up to a point. If you lock up at the rear it will lock both with ease and you'll spin.
More preload = more oversteer while maintaining speed. Doesn't seem to affect things that much though unlike the other 2.
Also, nice guide Keith, should help some people out who aren't sure what to change