Normally,I would agree about the tailgate having most of the residue. However, we had a similar problem with our Cadillac CTS in the past, and it was a defective crankcase ventilation system ( did not have a PCV valve), which I fixed myself. So I was a little apprehensive.
In a past life I was the Parts Manager for our local Jeep Dealership, so I am aware of the purpose behind TSB's.
In this instance, it's starting to appear that the dealership sprayed the wheels with some sort of conditioner that slung off going down the road, and attracted dirt, and left specks all down both sides of the car. I washed it off with a high pressure hose at a car wash, and so far it has not returned.