97 S420 Rear Differential Issues

[wlk1997]

Took my 97 S420 in for 90,000 mile service and have noticed a whining sound coming from the rear for quite some time. At first I thought it sound more like bad tires. The Mercedes Service Rep informed me it was my rear diff. He told me, "I would not worry about." I consulted a friend, who I think has some experience in mechanics and he also told me the same, however, he also informed me about Hypoid Gear Oil, it will make a difference. Is this an expesive repair or replace? Although the car still rides very nice, that whining sound really annoys me! Help!!

Thanks

wlk1997