The latest update: the police went to the residence of the owner of the truck. No one was home, or else they refused to answer the door, but the police spotted the quarter-pipe skateboard ramp on the property and confiscated it. It has been returned to my neighbor. The truck owner now has a warrant for his arrest.
It's a shame the police didn't search his premises (I bet they would have found some more stolen items), but I can only assume they couldn't justify a search warrant. So it's not an entirely satisfactory conclusion, but at least my neighbor has his property back.