UPDATE:
With the knowledge I gained from this forum, I tried the local GMC dealership one more time. This time, and with a different employee, they acknowledged a reported problem and advised that GM engineers are investigating, and they offered to replace the AMPLIFIER. The employee said another customer with the same vehicle had all the problems I reported and replacing the amplifier solved his issues.
A new amplifier was installed in my Yukon yesterday. No problems since and a much improved range. I’ll update the group if any issues return.