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The phase of the driveshaft is still important and having three joints does not affect the accel/decel properties whatsoever. It still occurs. In fact it is potentially worse with an odd number of joints and the only thing preventing that in the Volvo is the fact that the "odd" joint does not actually flex out of the drive-shaft line. Yet another reason why a failing center bearing or support is felt as such an amplified vibration.
As alluded to, many cars with both long travel suspensions and "solid" driveshafts exist. They have a splined segment at one end of the shaft to take up length changes as the suspension travels.
I think Volvo did it for safety reasons. Two shorter shafts with a hinge in the middle can flex on impact unlike a one-piece unit. This will help the engine/tranny to be pushed under the car instead of potentially jack-knifeing it into the passenger compartment (although I've never heard of such a thing I suppose it could happen). Incidentally (or perhaps not so), two shorter shafts can use thinner tubing and not collapse under the torque loading... Of course having extra flanges and u-joints kills any advantage.
There is a performance inprovement as well. The shorter shaft is less unsprung weight for the rear springs and shocks to deal with.
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Mike!
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