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
	}
}

It depends how is your topology.

If you you a specific VS fo each server then you don't need to identify it.

But if you have one VS then you somehow need to know which server it is.

You can use IPs, headers, cookies, or whatever unique application may has.