Update: drove 40 miles into work today. Grumbling resonance/light vibration is most present between 55-65 mph in 4th and I feel it more through the accelerator pedal, clutch pedal when I press it in and in the shifter handle. Not really under seat or rear ward. Pushing clutch in, it’s still there but much reduced. Same when popping into neutral (greatly reduced). Centering more and more on somewhat worn tailshaft bushing.

As mentioned, roughness while holding revs north of 1800 rpm in neutral is present, but has been for quite a while since the new motor mounts were put in.

Other than that, car ran good on the way into work; all vitals are good, temp and pressure wise.