It's odd for the weight and energy of a moving van to leave markings on rocks, but not on the grass leading to those rocks.
What must be kept in mind is that this isn't just an open, grassy patch between that road and the rocks--in addition to somehow not leaving any tracks in the grass, the van would also have had to avoid hitting 1) the road sign, 2) the significantly deep gully/depression between the road and the river, 3) the trees to the driver's right, 3) the light pole to the driver's left.
This emergency vehicle, in fact, shows the exact point her van would have had to travel through:
http://content.kare11.com/photo/2016/03/17/Van recovery_1458228994009_989312_ver1.0.jpg
Ideally one would travel to that scene and take measurements; photos (and the perspectives implied by them) can be misleading.
As for the rock-scraping, you'd still have to have a reason to come back to that area, in the first place, to even find the rock-scraping. The question remains: Why did police return to that location? There may yet be a good explanation.
I certainly hope the Sheriff is doing due diligence and checking the Johnson-Christiansen residence for evidence of struggle and injury.