I have never had a vibration traceable to the bushing period. And yes I have had bushings wear out.
But I have had an engine-originating vibration traceable to too much ignition advance, that felt like a driveline vibration.
Think about what is happening engine wise;
running up to 42 mph and then cruising, brings the Vcan on line, which combines at 2400 with the mechanical plus initial to total a large number which while cruising at very light throttle; the engine may or may not be happy about. So maybe you get a bit of complaints which manifests as a slight vibration.
Then at the shift, the rpm drops to 1750, but to maintain the 42 mph, you have to increase the throttle opening. So, maybe now the engine is steadily detonating, or misfiring, and the vibration gets worse...
My Vcan kicks in 22/24 degrees, and not all engines will accept that with 20* of mechanical, totalling 42* at 1750 rpm.... but might be Ok with 48 total at 2400.
So the first thing I would do is disable the Vcan and back the timing down to something like 12* at idle, with no more than 24 at 1750; and if you are running a full-time source of vacuum to the Vcan, I would later return that to the spark-port which brings it in at a later throttle opening.
So now, the engine will be starved for timing. If the vibration goes away, hallelujah you found it. But if it doesn't, well then you can put it all back together and sleep better at night.
Good luck!
