Rear Diff Hum

I think the rear diff is badly worn on my A4 V6 TDI quattro (2002)

its only done 78k - owned only by myself

I've checked the wheel bearings, driveshafts and diff seals, diff fluid level is ok

the rear end of the car hums loudly between 20-50mph

jacking the car up the rear wheels move nicely, quietly and smoothly

is this a known problem at all? I'm well out of warranty now, and although removing the diff is well within my means, I haven't got a lift so it will be a back breaking job. Anyone know of any nice diff rebuilders or any idea of a price?