Virtual server with two different web servers

I tried using the below irule, and it still doesn't seem to be re-writing it. I would prefer to have only one outside public domain, but at this point I'll do two if needed. Naturally I am under a time table to have this site operational by next Friday, so no pressure.

when HTTP_RESPONSE { if { [HTTP::is_redirect] }{ HTTP::header replace Location [string map {"A.internal.com" "X.external.com"} [HTTP::header Location]] } }