If it comforts you any, rear end collisions are among the rarest of crash types. Severe rear end crashes are even rarer still statistically speaking. In a typical rear end collision, the vehicles involved are going in the same direction, and the vehicle in front tends to be pushed forward to some extent upon impact to disperse the crash forces, rather than totally absorbing the entire energy of the crash. Frontal crashes are the most common, and side impacts are among the most injurious or worse.
Neither NHTSA nor the IIHS conduct crash tests to evaluate the structural rear end/crumple zone's performance and protection of the occupants in a rear end collision to my knowledge. The closest you can get to rear-end crash safety ratings for vehicles is the IIHS rear collision head restraint ratings, which mainly assess the vehicle head rest geometry. This evaluation hasn't been announced by the IIHS for the '08 xB, but here's the IIHS link for rear-end evaluation of the 04-06 Scion xB:
http://www.iihs.org/ratings/head_restraints/headrestraints.aspx?scion which also describes the nature of the testing.
I honestly didn't notice if the head restraints differed in the '08 model I recently checked out at the dealership compared to the previous model. I only noticed that either version had head rests in every seating position. The new model appeared to have a deeper cargo area/crumple zone behind the rear seat, although I haven't measured or compared the specs to be certain. It's one of the cars with sufficient head room for my DH that he's pondering the new model as a replacement for his car, if I don't overrule it if and when there are side impact crash test results from the IIHS (who has a tougher side impact test than NHTSA). The original model's lack of curtain airbags and poor IIHS side impact scores scared me off.