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Does the vibration change with vehicle speed?
Does it feel like the shake is in the steering wheel or in the seat?
If the vibration is the the seat then there are two likely causes:
The first would be a tire issue in one of the rear tires. This can be pinpointed by rotating the back tires to the front. If the shake moves to the front then the tire/wheel is definitely the problem.
The other option is a bent or unbalanced driveshaft. The dealer should be able to pinpoint that using a run out gauge.
Is it worse when lightly accelerating?
I would definitely suspect a tire issue in that case. Have a shop swap them from front to back and see if the vibration moves to the steering wheel.