Some owners of 2016-2018 Cadillac XTS vehicles may comment that a vibration is felt under heavy acceleration at around 20 mph, which can be felt in the seat. The condition may be caused by a second order vibration from the cardan joint. 

If the vehicle has been validated to have a driveline shudder under heavy acceleration at around 20 mph, install 4.5 mm-thick shims on top of the front RDM isolator bushings. 

Raise the vehicle. Remove one of the front differential bolts. Loosen the opposite  side front differential mount bolt. Position the flat washer over the existing differential mount washer at the location where the bolt was removed. 

Loosely install that bolt enough to support the differential. Remove the opposite bolt and install a 4.5 mm-thick washer on top of the existing washer and install the bolt. 

Tighten both bolts to specification. The new 4.5 mm-thick washers are available as P/N 11562546.