It does sort of sound like torque converter shudder. It happens when the TC can’t quite decide to either be locked or unlocked. Usually the symptom is noticed most around 35-55 mph, more often when the speed is changing rather than at a steady speed. If you trust the shop’s diagnosis, you could just replace the TC and see if that fixes the problem. None of the symptoms would necessarily mean the transmission is the problem. It could be, but you might get lucky and get it fixed just by replacing the TC. That’s probably what I do in this situation.