Create an iRule that can search and replace

Assuming that you can identify server1 and 2 based on IP you can use below exmaple

when HTTP_REQUEST 
{
	switch [IP::client_addr] 
	{
        "server1_IP" 
		{
            set server "SERVER1"
			HTTP::collect
        }
        "server2_IP" 
		{
            set server "SERVER2"
			HTTP::collect
        }
	}
}

when HTTP_REQUEST_DATA 
{
	if {[info exists server]} 
	{
		set payload [HTTP::payload]
		set new_header "<soapenv:Header><job:roleIdentifier>$server</job:roleIdentifier></soapenv:Header>"
		regsub {<soapenv:Header/>} $payload $new_header payload
		HTTP::payload replace 0 [string length [HTTP::payload]] $payload
	}
}