Bad differential bearings on a '99 355 ?

A friend's 355 is in the shop for 10K service and diagnosis of a speed-sensitive "whap-whap-whap" sound from the rear of the car that he originally thought was maybe a bad tire. After new tires were installed (needed them anyway), the sound persisted. The shop replaced a wheel bearing that, upon removal, was determined to be bad, replaced, yet the noise persists. Now the belief is the differential bearing could be bad. The "book" time for R & R is 60 hours. Yes, 60 hours! Not to mention, the bearing is probably around a grand. Any thoughts, similar experiences? For relatively low mileage, the bearings should last a little longer than that, shouldn't they? Especially with the sealed bottoms of a 355. Car has approx 12,000 miles, 6-speed. Thanks in advance.