Transmission Replacement ---> Weird Vibrations

Hey everyone, so I had another thread going regarding diagnosing and then replacing the transmission in my 2001 3.4L 4x4 automatic double cab (Linked below). Long story short the overdrive planetary failed and I ended up buying a rebuilt transmission and torque converter from Yota1 performance in California. After getting it installed I'm having some issues, so in hopes to get some more eyes on this I'll describe it all more concisely here.



https://www.tacomaworld.com/threads/overdrive-transmission-failure-replacement-more-problems.840813/

After I installed the new trans the truck had a significant vibration only when in park and neutral. It can only be felt between 900-1300 and 2200 RPM+. Completely smooth in any other situation. I've driven up to 60 mph and everything felt really good. Strangely if I shift into neutral while moving it's not there. The frequency definitely moves with the rpm. I overfilled the trans fluid a little at first but draining it to the right level had no impact.

At first I thought it was misfiring but further diagnostics have led me to believe that's not the case as there is no CEL and the fact that it doesn't happen under load. In an attempt to isolate the problem I unbolted the torque converter from the engine and pushed it back. This got rid of the vibration completely. I bolted it back up being sure to start with the centering bolt and torque it all properly, also offsetting it from where it was before. The vibration came back just as bad as before. This has me thinking that it's something with the new transmission or torque converter. What's rotating when I'm in park/neutral that could cause this? I would think just the torque converter and the pump, but why would it go away when I'm moving or in gear?

Now there's definitely a chance it's something else, but I'm not sure what it could be. I'd never replaced a transmission but I followed the service manual and it all went very smoothly and lined up well. Here's some of the other things I did while the truck was down: spark plugs, fuel filter, trans cooler, new radiator, new trans mount, rear main seal, and some new gauges (trans temp, oil pressure, voltage). I don't think any of that would cause this but maybe.

I talked with Yota1 and they wanted me to send back the torque converter along with my old one so they could compare them. I did notice that the new converter was heavier than the old one, even before I filled it with fluid. However, they did not seem especially confident that would fix it and didn't seem to have much more of an idea than I do about what's causing this. Before I have to take the transmission off again and deal with sending things back and forth I am hoping someone can help me make sense of all of this. Thanks in advance!