Aside from a natural desire not to have another superpower's military right on your borders (remember the Cuban Missile Crisis?), I think the main reason China doesn't want a unified Korea is that their big near-term ambition is to replace the US as the dominant Naval power in the region. They're also trying to expand offshore territorial boundaries to lock up mineral and fishing rights, like those artificial island projects in the South China Sea.
A unified Korea might mean an even larger US Navy presence with access to more Korean ports, along with conflicts involving mineral and fishing rights with a unified Korea. So it may be more about what's happening offshore, than worrying about US tanks on the border with China.