The Giants have a shortstop. He's very good, and his name is Brandon Crawford. They do not need to trade for an oft-injured, expensive shortstop like Troy Tulowitzki. They do not have the prospects to trade for him. Even if they had the prospects and the money, it would make more sense for them to allocate those resources to the rotation.
But here's a rumor that connects Troy Tulowitzki and the Giants that I'm posting only for traffic. From Jon Heyman at CBS:
Perhaps the true reality is that Tulowitzki wants to be traded.