Just wondering if you got to the bottom of this. I just had my Goodyear polyglas tires road force balanced and got some improvement. The increased rpm may bring out a tire balance issue that you didn't have previously. Let us know what you have found.
The sensation/issue is still there. The front slip yoke does have movement, but not a ton and it has not worsened.
I have put about 500 additional miles on the car since my last post. No change or increase in severity. What I have noticed is what seems to be a decrease in the grumble, or at least less noticeable, above 65 mph or 2400 rpm in 4th/OD. After that point, the light vibration/grumble seems much less pronounced. Starting to wonder more about the trans mount, too hard of motor mounts, or a bearing in the trans making some commotion at specific RPM in 4th/OD.
