Just wanted to update. Same existing problem still exists. Driveline angles are even closer now. Got brave the past few times and brought the revs up on the highway in 4th/OD to 2600-2700, which is about 72-73 mph. Oddly, the grumbling minor vibration seems to smooth out some at that speed and it is substantially quieter; not gone, but quieter.
With that said, I am beginning to wonder if the newish motor mounts are just subpar at dampening noise and vibration, and/or the same for the trans mount. Other than that, the only other feasible potential cause in my mind for the issue (which is most present from 2100-2500 rpm in 4th/OD) is the tailshaft bushing.
Who knows, maybe in the end, with everything checked off of the list, there might always be some of this noise/roughness present at the noted engine speed in 4th/OD with this A833 OD trans and the associated 3.92 gear. I put 250 miles on the car this past weekend, half being highway miles, and the severity never varied or changed.
