Replace Source ip with Real Client IP

Give this a try. One caveat, this will only work if your LTM is the default route for the backend servers, otherwise you will end up with asynchronous routing.

when HTTP_REQUEST {
    if {[HTTP::header exists "X-Real-IP"]}{
        snat [HTTP::header "X-Real-IP"]
    }
    elseif {[HTTP::header exists "X-Forwarded-For"}{
        snat [HTTP::header "X-Forwarded-For"
    }
    else {
        return
    }
}