Beautiful work and updates!
Is it possible the subframe itself is flexing under heavier throttle, when the vehicle is moving? Maybe a GoPro or some cheap alternative camera mounted around that area could shed some light on what's going on.

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Current:
2021 Charger Scat Pack Widebody "Sinabee"
2018 Durango R/T

Previous:
1972 280SE 4.5
2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
1999 Chrysler 300M - RIP @ 221k