It’s a bit of a work-around, but you could create a smart group that just locates indexed documents, then sort those results using the column Path in the Three Pane view.
Once you have this list of documents, how do you plan on moving the documents without breaking the reference to them in the database?
First I want to move the items in ~/Documents into a group in the database, which is the indexed folder ~/Dropbox/Documents. And then I want to move these items into the external folder.
That will be pretty simple to do, assuming that there is not a complex folder structure contained in ~/Documents/ and ~/Dropbox/Documents/. If there is, it becomes more tiresome to move everything over. I did something similar using iCloud Drive, and it was made simple by the fact that I only index folders, and by extension all the documents they contain, and all my indexed folders originate at the root level of the database.