In answer to your question, Supermoose... YES, you need to match the ratio of the rear diff with that of your tranny/front diff. If they are different it will be very bad. If your rear diff does come in the wrong ratio it's fairly easy to change it with a new ring/pinion set. That said, it's also VERY easy to get a US 4.11 rear diff.... I see no advantage to getting a 'JDM' one either... they're the same thing.