There is no real alignment adjustment for the rear of your car. The toe is fixed, and while You can loosen the rear axle beam mountings, and move it around for cross adjustments, but there is no "wear/factory" type adjustment, if that makes sense. The shocks are your most likely culprit. Also, regular tire rotations will usually keep this type of wear at bay. i have never had this issue with my golf (essentially the same chassis as your beetle), and i rotate my tires every 5,000 miles with the oil change, and drive up and down the mountain roads daily, sliding like a little playful boy. The last set of tires went almost 45000 milles, those were cheapees, and i had perfectly flat wear on them. So, to recap.....
shocks, then rotate regularly, and don't call me in the morning.
And, air pressures. watch the air pressures as well as they can accelerate the already present wear.
