One problem related to driveshaft has been reported for the 1999 Audi A6. The most recently reported issues are listed below.
We have an 1999 Audi A6 quattro with only 35 ,000 miles on the odometer. Recently discovered an oil leak under vehicle, found out from Audi service that the prop shaft/transmission seal is leaking. I questioned the service rep and he said, he's seen it happen on others with 20,000 miles on the cars.