Set variable from <input> value

I need to implement Google Analytics (Universal/analytics.js) on a 20-something part AJAX-based questionnaire. The questionnaire works something like a choose-your-own-adventure, whereas the follow-up questions are all determined by the preceding answers.

Each input has a name and an ID. I'm interested in pulling the value from this field and setting it as a global variable in a dataLayer. Everything has a unique ID and Name, which I've started collecting in a database.

Here's an example of the HTML:

<table id="ctl00_ContentPlaceHolder1_2_Table1" class="CompSSRadio" cellpadding="1" cellspacing="1">
			<tr>
				<td class="CompSSRadioLabel">
				</td>
			</tr>
			<tr>
				<td class="CompSSRadioResponses">
				<table id="ctl00_ContentPlaceHolder1_2_1_48_1" class="CompSSRadioResponses" border="0">
					<tr>
						<td><input id="ctl00_ContentPlaceHolder1_2_1_48_1_0" type="radio" name="ctl00$ContentPlaceHolder1$2$1_48_1" value="4" /><label for="ctl00_ContentPlaceHolder1_2_1_48_1_0"><SPAN style="FONT-FAMILY: Museo 500"><SPAN style="FONT-SIZE: 14pt"><SPAN style="FONT-FAMILY: Museo 500"></SPAN>Test answer 1</SPAN></SPAN></label></td>
					</tr><tr>
						<td><input id="ctl00_ContentPlaceHolder1_2_1_48_1_1" type="radio" name="ctl00$ContentPlaceHolder1$2$1_48_1" value="5" /><label for="ctl00_ContentPlaceHolder1_2_1_48_1_1"><SPAN style="FONT-FAMILY: Museo 500"><SPAN style="FONT-SIZE: 14pt"><SPAN style="FONT-FAMILY: Museo 500"></SPAN>Test answer 2</SPAN></SPAN></label></td>
					</tr>
				</table>
			</td>
			</tr>
		</table>

Here is the "next" button:

<input type="image" name="ctl00$ContentPlaceHolder1$btnNext2" id="ctl00_ContentPlaceHolder1_btnNext2" src="../App_Themes/Images/Right.gif" onclick="javascript: pageTracker._trackPageview('//forward.html');window.document.getElementById('ctl00_ContentPlaceHolder1_btnNext').disabled=true;window.document.getElementById('ctl00_ContentPlaceHolder1_btnNext2').disabled=true;__doPostBack('ctl00$ContentPlaceHolder1$btnNext2','');" style="border-width:0px;" />

And here is a script which does something with the form info:

<script type="text/javascript">
//<![CDATA[
var theForm = document.forms['aspnetForm'];
if (!theForm) {
    theForm = document.aspnetForm;
}
function __doPostBack(eventTarget, eventArgument) {
    if (!theForm.onsubmit || (theForm.onsubmit() != false)) {
        theForm.__EVENTTARGET.value = eventTarget;
        theForm.__EVENTARGUMENT.value = eventArgument;
        theForm.submit();
    }
}
//]]>
</script>

I don't know if I need to hook into that or anything, since that's what's submitting information. Obviously I'm clueless. I feel like this is a fairly simple task, it's just my lack of javascript knowledge preventing me from crafting the necessary script.

Thanks ahead of time!