I've done some experimenting recently on my truck. I think I have the same vibration.... Under light throttle around 2000-2300 rpm I fell the vibration in floor, gas pedal and steering wheel.
I used a vibration analysis app for my phone while holding my phone against the steering wheel while creating the vibration. It showed a frequency of about 133hz.
At 2000 rpm, first order vibration is 33hz. So 133 is basically a 4th order vibration... On a v8 engine 4th order is firing frequency. A good indication that the engine firing is exciting some other vibration.
Doing more research lead me to the fact that the diesel trucks get 3 extra mass dampers: 1 on the steering wheel, 1 on the back of the transfer case and 1 attached at the passenger differential mount
My theory is that those dampers will help the gassers too. I believe the vibration is "coming in" to the cab from the steering shaft which is linked to the frame. I have high hopes that the steering damper will help the issues but I do notice my issue gets worse with the truck in any form of 4wd so I think the diff damper might be necessary as well. The xfer case damper requires a different case half so that one isn't easily addable. The diff one can be bought but you need to remove the mounting bolts to install it. I already have a used steering wheel damper ready to go but haven't had the time to install it. I will report back my findings once I do.
Very late edit: do you now have a 2019 5th gen truck or a 2019 "classic" (classic being the same body style as the 18 it replaced)
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