ughoavgfhw offered the correct answer (above). Use Cmd+E on the selected text to begin searching with that text. Then immediately type Cmd+Shift+F to search the whole project for all instances of the text hit enter.
This is quite quick, consistent, and sensible. It’s not as immediate as Visual Studio’s single-stroke Find/Find-in-Files—Xcode requires two strokes rather than one—but personally the extra step doesn’t bother me.