Success!
Thanks to all of you who provided input. I would have been one to suggest checking the pin locations as well, but I was super careful in putting the connector together and double and triple checked each connection.
The prize goes to Ken at Garmin support (I also asked the question directly to Garmin). His response started the same as everyone else's, but then he went in a direction that I hadn't thought of. His response was, and I quote:
1. Verify USB port terminations at connector.
2. USB cable length should not be over 4 feet.
3. Use a good quality USB cable.
What? USB cable no longer than 4', I had never heard that before. I was using a 15' Amazon Basics cable. I've never had problems with Amazon Basics products before, but this one did not work, either due to the length or just poor quality.
I stopped by Wally World on the way to the airport and got a 6' cable (couldn't find a 3'). I plugged it in and then the GDL 8X Install Tool kept crashing. Oh great, another problem. After a couple of computer reboots I got a connection and it was rock solid throughout the entire ground test process.
It is interesting because when I was finished I thought, you know, when all else fails RTFM (read the instructions). And on page 7-2 of the install manual, item 3 at the top of the page says "The cable length should not exceed 16 ft." I don't know if the problem was that the cable was too long, according to the installation manual it shouldn't be, but per Ken's suggestion it definitely was, or if it is just a poor quality cable.
Either way, everything appears to work fine, no faults, no interference, all green. All that is left is to put everything back together and go fly it. Yeah, right, ALL that's left...