I always assumed that the reason for the shaft beaing able to collaspe was so it could be removed and installed on the vehicle especially because the Centering Bearings on the Flex Disc ends need to be cleared.

Before the Drive Shaft Collar is tightend the Car is supposed to be put on all 4 Wheels and rolled back and forth to adjust the Shaft Length and then the Collar is supposed to be tightened to a specific Torque.

With the Collar Tightened all of the side play in the Splines is closed and that keeps the Splined from wearing.

During th rebuild they likely tightend The Collar.
Also the sections of the Drive Shaft are supposed to be match marked.

In the Picture te Flex Disc is remved and yo can see the Centering Bearing on the righ and the Rod that stick out that goes inside of the Centering Bearing on the left.
You need to collaspe the Drive Shaft enough to get that Rod out of the Centering Bearing so the Drive Shaft can be moved sideways.

Front and Rear had the Same Rod and Centering Bearing.

The Centering Bearing inside diameter needs to be checked for roundness and it needs Grease.

Attached Thumbnails

 

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84 300D, 82 Volvo 244Gl Diesel