1. In Outlook, right-click on the top of your mailbox in the folder list (on your email address) in the pane on the left and choose ‘Data File Properties.’  This window should open:
  2. Click the Advanced button in the bottom section:
  3. Click the Advanced tab:
  4. In the Cached Exchange Mode Settings section, uncheck the box for ‘Download shared folders:'
  5. Click Apply and/or OK until all the dialog boxes are closed and you’re back in Outlook.
  6. NOTE:  You must close and reopen Outlook for this change to take effect.

That’s it!  You should now be able to search successfully within the shared mailbox.

A note on performance:  by making this change, you will not be running the shared mailbox in Cached Exchange Mode, where your computer keeps a copy of the mailbox on your local hard drive.  As a result, you may notice an adverse effect on performance, since Outlook is now accessing the shared mailbox directly.  If this happens and you want to speed things up, you should undo this change.