I had an issue with my transfer case as well. Wasn't sure that is what it was until I saw another post on here describing the same issues that i had. I have a 2016 Cayenne. When i would accelerate at around 30MPH, the car would start to feel as if i was driving over cobblestones and shaking. This would only occur once the car was warmed up also. I took it to the Porsche dealer and after test driving for 30 seconds, they confirmed the issue. Luckily for me, this is a know issue and Porsche extended the warranty on the transfer case for 7 years, so it was covered.
Fortunately (and unfortunately for you) the Series-1 (955/957) were not known to have transfer case issues besides what might be considered normal or expected failures (a rather low percentage of issues.) The Series-2 (958) does have issues and Porsche extended the warranty on the transfer cases to cover people who have them fail. The letter from Porsche for the 958 doesn't cover anything "for life" - it extends the warranty by 11 years for the early production 985 (most of which are past 11 years now) and 7 years for later model 958's. Dan, I think it might be worthwhile to re-read that letter.
To the original poster - sorry - there is no service campaign, extended warranty or recall on any of the 955/957 transfer cases.