Air bags mounted within the rear springs:
Some part numbers in post #9
http://forums.tdiclub.com/showthread.php?t=358338
Also try search terms "air lift", " "air bags" AND "towing" "
Air shocks are another approach to read up on. I'm not familiar with the pros and cons of either yet.
http://forums.tdiclub.com/showthread.php?t=363379&highlight=ghetto