Solved!!

Here is what worked for me.

1) Called the hosting company and asked them to enable cURL.

2) Set my HD5 Hash value on the Authorize.net account to match the value in the Authorize.net settings in OpenCart.

3) Created a new catelog/controller/payment/authorizenet_aim.php file using the code from:

https://github.com/qphoria/opencart/blob/89c31c93301fbdf448d1a89ad63ffadce460d5c8/upload/catalog/con...

4) Replaced the original authorizenet_aim.php with the new authorizenet_aim.php file on the server.

5) Set OpenCart system->system->settings->option->Order Status to Pending

6) Set Payment->Authorize.net (AIM)->Order Status to Pending

7) Tested in live mode.

 I hope this can help another OpenCart / Authorize.net user.