CRM - Cross Domain Functionality Through F5

This is a fairly common issues and you have a couple options. You could change the headers on the fly with something like the following:

when HTTP_RESPONSE {
  if {
       [HTTP::is_redirect] &&
       [HTTP::header Location] == "partners.f5lab.com"
       } {
    HTTP::header replace Location [string map {"internal.crm.com" "external.crm.com"} [HTTP::header Location]]
  }
}

Or, you could ensure that you added an Access Mapping to SharePoint/CRM with the header its receiving on the default zone.