It would be unusual to have wear in a low mileage diff, but there may be some play at the top end of what's acceptable. If you jack the rear up you can test the free play between each wheel. Generally speaking, diffs have four thrust washers. There will be one between each planet gear and the carrier (either end of the pinion pin). The other two are between the halfshaft gears and carrier. When you take up the drive, the pinion loads the crownwheel, and those four small gears load up. This can produce a loud 'click'. MGB's are notorious for this, and the thrust washers can be changed on a Salisbury axle in a couple of hours with no special tools.
