Thanks for the suggestions, I had already done both of those things (aspnet_regiis -c and adding webresources.axd to the exclude list). I did them again for good measure and still not working.
As I was writing this post I thought of something else to try and it worked. For those of you out there having this problem, let me re-state what the problem is: Pages in your DNN site that use the WebForm_DoPostBackWithOptions javascript function and throw a Javascript error. From what I've seen this applies to 4.x versions of DNN that have been upgraded from an earlier version of 4.x, at least in my case it does. A lot has been said in this thread regarding webresources.axd and I've spent a lot of time reading up on what that file is and what it does. My best guess is that webresources.axd makes a call to the .js file containing the client side validation functions. The problem arises when your page also includes a src="WebUIValidation.js" reference. I did a fresh install on another server of DNN 4.4.1 and noticed that my page didn't include that reference, it only had the webresource.axd calls and thus no javascript errors.
To fix the problem on my original server I simply removed the aspnet_client folder from the directory in which my website files reside. This was just a guess so I did keep a copy of this folder in case I needed it later. But removing the aspnet_client folder and then restarting the application from the host settings page seemed to do the trick. I also cleared my browser cache and the application cache on the host settings page just in case. You might also have to restart your website in IIS as well. After all that is done, refresh the previously non-functioning page and it should work. If you look at the source of the page now you'll see that it no longer references WebUIValidation.js but it still performs the validation. However, it seems that now the validation is performed only server side, not client side & server side.
What I don't know is if this is the intended way of doing things for DNN. If you know anything about ASP.NET validation, you know that it can be performed both client side and server side. The advantage with client side validation is that it saves round trips to the server. User input errors can be caught before ever sending requests to the server. So does anybody know what is going on with this stuff?