This isn't a case of hitchhiking animals, but it fits my theme--wild animals interacting with things people make.
In Viroqua, Wisconsin, a man awoke to see his concrete lawn ornament--an elk--tipped over. Not far away, a seven-point buck lay dead with a fractured skull. The concrete elk weighed 640 pounds. You can see the story and photo here.
Update: One driver (from a reader response here) claimed that a deer rammed his car: "As the survivor of an auto-odocoileus collision several years ago, I must take exception with your lede’s reference to “cars slamming into deer,” which presumes such collisions to be the fault of the driver. My vehicle was hit, broadside, by a rogue ruminant who charged from more than 50 feet." Audentes, 1/30/07