You should be able to book that reward online; I've booked intra-US awards on the AP Web site before (if, of course, there is availability).

However, a couple of notes. First, you can only see availability on carriers other than AC/Jazz if you are logged into the Aeroplan site first. Secondly, you can't see a calendar of available days with partner carriers, so if there is no availablility on your preferred day of travel, you have to go back and search another day.

Additionally, if you are flying a short-haul route (within NA), it may not be worth it, as it costs 25,000 miles for any trip within North America on UA or US, but only 15,000 if there are classic seats left on an AC flight.