Dean, I think you are right. screen readers in general don't see the highlighted text. It has something to do with word itself. However, I have found a work around. Following steps can be used to easily navigate the highlighted text in a document:
1. Press Ctrl+H to open the Word's Find and Replace dialogue box.
2. Press Alt+M to extend the dialogue box. Nothing will happened if the box is already extended.
3. Press Alt+O to extend the format tab in the Find and Replace dialogue box.
4. Press H to toggle on highlights in the format tab.
5. Now, word is set to find the highlighted text in the document. Press Alt+F to find the next instance of the highlighted text, or manually tab to the Find Next button and hit enter.
6. Press ESC to exit the Find and Replace dialogue box. Page UP and Down commands can now be used for going backward and forward with the highlighted text in the document.
I have also recorded a macro for the above process. I will be happy to share it if anyone is interested.
** If you wish to restore the Page UP and Down functionality, simply Press Ctrl+G to open the Go To dialogue box, Press Shift+Tab to enter the Go To What list, and hit enter on the first entry, which is Page.