This is a pic of my U-joint. Though I was not having any vibration issues I had removed the Drive Shaft when I did the B2 Piston Change on the transmission and I found the sticky U-joint and replaced it.

Anyway in the pic you can see that the U-joint would not flex as it should.

Note that the Guy that claimed that you can free up a sticking U-joint claimed that the sticking was due to dried up grease. But, there was some rust in the bearing caps on mine.

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84 300D, 82 Volvo 244Gl Diesel