Doing it in ajax will complicate it a bit more, and Iframe is designed to take a page and execute it in a isolated frame.
Ajax will take code and insert it into a specified block.
best way will be to change your aspx pages into ascx controls and then change your iframe into a panel and
Use a ajax panel and on tab item click dynamically add the user control to the panel/div.
Also if you are creating a commercial website for search engine optimization note that using javascript/ajax is not the best way to navigate content you wish to make available to search engines because search engines don't execute javascript. and linking to a javascript transition using urls , well i havent seen a single search engine capable of doing that.