you should consider a couple things here.
First is minor, telerik has a 'modernizer'-like helpers at its scope.. so "$telerik.isIE" will hold the boolean value.for determining whether the user-agent is Internet Explorer.
Second, instead of falling back to legacy methods, in particular: ActiveX; use and implement Polyfillers and shims; that is what they are for. In which case, you wouldn't even need to handle if isIE, isChrome, IsFirefox clauses; the Polyfiller and shim libraries would 'describe' and inject the support and functionality of modern browsers, and features like xhr.
Third, consider turning 'loadAjax' into a nested function of 'loadDiv' and defining (dimensions) the variable (named xhr) in the loadDiv function scope. At that point, you will not need to pass 'div' (which is really an ID), nor 'xhr' to load Ajax, because it will be in the scope of it's parent function.
xhr.onreadystatechance = loadAjax;
el = document.getElementById(id);
el.innerHTML = xhr.responseText;
note: I didn't add the readystate or status checks within the readystatechange for simplicity.. you'll want to have those.