What is making you think that pega.u.d.asyncRequest is not doing asynchronous processing? Do you have a custom AJAX call in your configuration that is using pega.u.d.asyncRequest or just normal OTB functionality? What version of PRPC are you using?
At a the lower level the pega.util.Connect.asyncRequest can't even be made to be synchronous, there is no parameters or options etc. The connection is always set to a asynchronous (true):
o.conn.open(method, url, true);
What you might be seeing is how the server is handling the requests but I need more information to discuss that further.