How are these dropdowns and radio  buttons being created? Do you have access
to their ID and/or name attribute(s)?

Essentially, you'll need to add an event handler to the dropdown  list that
catches the onchange event and passes a reference to the DDL, e.g.

onchange="changeRadioButton(this)"

Then in a Javascript function -- provided you have the name of the radio
button list or the ID of one of them -- you could do something like this:

function changeRadioButton(oList) {
var sRbID = sRadioID;
var sRbName = document.getElementById(sRbID).name;
var oForm = document.forms[0];
// if you have more than one form, you can probably get the reference
through a control
var aRadioList = oForm.elements[sRbName];

aRadioList[oList.selectedIndex].checked = true;

}

Let me know if this helps or if I missed your point entirely.