It depends on which ones are bad. By rights, if one or two are seeping the others can't be far behind so You are better off to do them all. That means pulling the motor. There is a tool that allows you to change them with the block in the car (I belive Snap-On makes it) but if you have one behind a motor mount you will have to at least lift that side to get the mount out.
Actually, getting them out isn't nearly as hard as getting the new ones back in, especially those right behind the motor mounts because there isn't any room to swing a hammer to seat them. You are going to have to lift the motor anyway, might as well pull it and seat them properly.