I think there is some type of driveshaft fix that is very common, and I think possibly some sort of plastic hose problem. By 100,000, both of those have probably been fixed already.
I see lots of Cayennes at every DE I go to; folks use them for their tow vehicles. There are several in town here (and I live in a small town)
Rennlist has a pretty big Cayenne forum here:
Rennlist.com
There is also a Cayenne technical articles section at Pelican Parts, similar to the 101 Projects for the Boxster:
Porsche Cayenne Technical Articles - Porsche 955 (2003-2008) | Pelican Parts
Here is one description of the driveshaft issue:
Cayenne Drive Shaft Clamping Bearing Support Mount: Drivetrain & Drive Shafts
Edit: I found the pelican article for this issue, don't know why I didn't find it before:
http://www.pelicanparts.com/techarticles/Porsche-Cayenne/29-SUSPEN-Driveshaft_Bearing_Support_and_Flex_Disc_Replaceme nt.htm
If I didn't already have a Toyota FJ Cruiser I would be looking at one of these, especially after the recent Panorama article about them.
Good luck!
Steve