I don't think there is a way to include POST data on a redirect as a redirect is just a HTTP response code with a url as a parameter.
If you could provide your network layout along with how you want the HTTP traffic to flow. And add the current iRule you are using....
If all you want to do is to accept traffic for a domain and change that domain when it goes to the backend server, just change the host header and continue the request to the backend server.
Again, if you pass along your config, ins and outs, and your current iRule that will help us help you.
-Joe
Nimbostratus