This sounds normal to me, don't forget that in a de-dion car the diff casing is bolted to the chassis and it's very close to your left hip whereas in a live-axled car it isn't, hence you are less likely to notice any backlash on a live-axled car.
Have chat with the people at your local meet, go out in their cars, take someone out in yours, you'll soon discover whether yours is unusual.