You need to undo the drive shaft at each end and rotate the U joints and inspect for binding. When you accelerate/decelerate the angles of the shaft's ends change and the u joints binding will give shudders. The bushings in the rear lower arms may also be in need of replacement making the shudder worse. If you do any work on driveshaft find the alignment marks where front and rear shafts slip together or make your own. The shft is factory balanced and phased and not putting it back together the way it came apart will also give you quivers.