Get values from client side to server side using JQuery.

Declare 2 hidden field in client side :

<input type="hidden" value="0" id="hfUId" />
<asp:HiddenField ID="HiddenFieldUId" runat="server" />

Get the values of response.

$j.ajax({
type: 'POST',
url: '/TestService/TestService.asmx/GetUserDetails',
data: { userId: UserId},
dataType: 'text',
async: false,
 success: function(response) {

//If response has multiple values then use split function
var splitData = response.toString().split("$");
splitUserId = splitData[0];
 //Store values of response in hidden field
 $j('#hfUId').val(splitUserId);
 var data = $j('#hfUId').val();
SetData(data);
 },
});

function SetData(data1) {
document.getElementById('<%=HiddenFieldUId.ClientID%>').value = data1;
}

Now, You can access the value of HiddenFieldUId in server side as HiddenFieldUId.Value