You feel it in your butt/seat/chassis. Steering wheel doesn't move.
Once at the road speed where it occurs, turning off OD has zero effect. So that rules out engine and transmission rotating assemblies
It's a unibody, so no cab mounts (good call though). Recently re-re-relocated the exhaust in the weird possibility it was hitting somewhere causing a resonance. I can't see anything remotely close to hitting at ride height or while "powerbraking".
