Be aware that even slight changes in the center bearing support height can cause vibrations at low speed. Volvo issued this TSB:

"Driveline Take-Off Vibration: Shimming May Be Required. From Volvo Technical Service Bulletin 45/102 Jan '91. [Symptom:] Take-off vibrations at low speed, especially with high acceleration and/or heavy load, can occur in some cases. [Solution:] The level of vibration can be reduced by changing the vertical position of the center support bearing. Best results differ from car to car. Start with a change of 6mm (1/4") upwards. If results are not acceptable, try positions from 5mm (3/16") downwards to 12mm (1/2") upwards from the original position. Note that clearance between drive shaft and fuel lines must be at least 20mm, and between drive shaft and the fuel tank at least 16mm. Shimming upwards should not exceed 12mm (1/2") and after re-positioning the center support bearing housing should not contact the floor pan."

In addition to what Most Unusual mentioned about the importance of preserving the driveshaft position relative to the yoke, it's also important to put the flange bolts back in the same holes. This is because Volvo, at the factory, sometimes used flange bolts of different lengths to get the overall balance right.