the 120 has a lot of suspension travel in the back. The gives it better road holding abilities, especially on rough roads (when the 120 were designed, 90% of Sweden's roads were unpaved). A one peice shaft would severely limit the suspension travel, especially given that there is little distance between the end of the transmission and differential flange. If you think about it, trucks get away with a one peice as the movement of the suspension has a longer tangent relative to the end of the transmission, u-joints range of motion doesn't allow then to rotate freely past a certain point, hence the 3 of them.