You can usually have custom axles made for your application. For example, if you have a Chevy that you're putting a Ford 9 inch in, you can get axles made with the Chevy bolt pattern for that rear end.
What you want to do is to contact some place that deals with differentials and ask them about your specific application, and see if they can make axles for the older applications. That is an excellent question.
I wish I had a 4.11 or taller rear end in my 3/4 ton. I have a 4.56, so it's only got about a 60 mph comfortable cruising speed and about a 72 mph top speed.
