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.