Is there a way you could make it so the soon to be replaced div's dimensions are saved so that the page doesn't collapse while loading the new div? Also, could you add a feature to disable scrolling to the top of the page?
The Ajax For All plugin currently offers these features so maybe you could use some of that code. I tried to dissect the code myself but after 10 hours I didn't get very far.
EDIT: My navbar tags aren't updating even though the URL is updating. How can I fix this?