GET Request using Host Header Value not IP Address

I can access the site if I use the IP of either sharepoint front end in my host file.

Does this mean you're going around the F5? If so, whatever you have in SharePoint's AAM configuration is what you need in the Host header. The easiest thing would probably be to add the external name to AAM, but in lieu of that, the above iRule needs to replace the host header with a name that the SharePoint server likes. If you have multiple SharePoint servers with their own AAM settings, then you'd also need a way to switch these values in the iRule based on the load balanced server IP.