Finally an update.
The dealership had my truck since Monday morning. On Tuesday evening, I called them asking for a status update. They said they'd have an update for me later that evening. I never got that update. So I waited until Thursday morning, and called. What do you expect: I got the same old line. They assigned it to a tech, who "could not replicate the issue". So they assigned it to the shop foreman, who also "could not replicate the issue".
Incredulously, I said to them that it is 10000000% easily replicated, each and every time, and offered to come to the dealership again to show them. They agreed, and the shop foreman came along for the ride. Within 30 seconds of me getting into the driver's seat, I reliably replicated the issue, and made sure the tech actually felt the shudder at least 3 times. I asked him if he wanted to bring the scanner so that we can do it again, and he said no need, he had all that he needed to proceed.
Fast forward to this evening, I called them for a status update, and the service advisor reported to me that the shop foreman tech finally indeed did find "codes on the TCM" and on the output graphs which indicate clear TCC slippage. The reasoning they gave for "not believing" what I was telling them is that they have never seen a 10 speed transmission fail, and so were completely caught by surprise that it was indeed the case.
They said based on the diagnosis that it was one of three things:
1. Solenoid/s
2. Valve body
3. Torque converter
They said since it was narrowed down to those 3, they would start doing a more in-depth diagnosis in that order (which is also in order of ascending cost) to determine which of the 3 are the problem, and then provide me with an estimate on repairs early next week.
In hindsight, if they would have just done what I asked them to do, they could have saved me a wasted week, because I literally told them what to do, and expected that it was TCC shudder. Whatever. That is behind me now, and I need to focus on getting this fixed.
@NickTransmissions, what are your thoughts on which of the three it might be? I'm really hoping it's either the solenoid or the valve body, because I know from experience that these two are much less expensive than the actual torque converter.