Irule to failover between two sites based on return string respone.

Oh wait... you only have one wide_ip, in this question. It uses a GTM pool of two virtual servers, in two LTMs, in two data centers. Correct?

You want GTM to concider the virtual servers down, not based on their status in the LTMs, but based on a new monitor, unrelated to the LTMs. An HTTP monitor, which would check the status based on the result of some URI.

You can achieve the later, I believe, only by reconfiguring your wide_ips to use "virtual servers" manually defined on "generic host" servers, as opposed to BigIP type "servers". Then use a cusom HTTP monitor, to monitor the defined virtual servers. But dont take my word on it, maybe you can "add" a custom monitor to the exisitng implicit LTM minitors, or maybe I did not understand you setup and your needs.