993 Box problem ?

[lanan]

I haven't had the car long, C2S 43k miles. So the other evening I gave it a squirt. Obviously I was not on the public highway as I was going over 70mph. On over run I noticed a vibration, which settled by time I was down to 80. Is this likely to be a CV joint/driveshaft or gearbox problem. Are there any known probs with 993 boxes.
What would the likely cost of a rebuild, relacing bearings and seals.

[slippydiff]

I seem to recall a similar post on PH recently, turned out to be shot CV joints I think.