The constant yelocity joint------a u-joint changes shft speed as it rotates, this is what causes driveline vibration when the u joints are at either odd or too much of an angle-
The cardan constant velocity joints solve/correct that----first ones I saw were on my 1970 Lincoln ------

every airbag install that I have seen has so much height travel that the short arms (4 link) rotate the rear housing so much that u joint/shaft angles are excessive and you get drive line vibration----