Heard what I thought was a front wheel bearing. Howling type noise at a specific speed that would dissipate if I unloaded the suspension on the right wheel. My indy (a very respected Porsche shop in Minnesota) is saying that they heard the noise coming from the rear diff. My question is, can the rear diff. only be replaced in a 2006 C4S? How bad of a job is it?

 

I just swapped in a quaife lsd for my 08 c4s manual.

Not sure on the labor price, as I don't have the final bill for everything yet. But I have a perfectly working stock oem diff for sale, if you need one.

 

Try changing the differential oil and see if the sound goes away. Use the Porsche specific oil. Easy drain and full procedure than can be diy if required.
also take a automotive stethoscope and double check if sound is from the diff.