Currently it is not possible to select two revisions on different wiki history pages to view the differences. So please make this possible.

You could add two text boxes near to the button 'View differences'. These get filled with the revisions that are selected by the revision radio buttons. And the values of these boxes are used to show the Diff page. The values may be changed by the user manually. Then it would be possible to view the difference between revisions 678 and 3. You could leave the existing pre-selection for each page as it currently is. The values would be changed to those revisions. But the user has always the chance to change these values.

I hope you get what I mean. At least I tried to suggest something that could be implemented easily and quickly. This is of course not the best solution (because the user has to remember the values if he browses the history pages), but it is probably better than no solution.