Thanks for the ideas. I wasn't thinking of starting the engine without a flywheel. The starter really winds out that way. If the motor itself is vibrating, it can only be something in the clutch / pressure plate or the fan, as Cletis suggests, unless something catastrophic has happened with a connecting rod in which case I would know I hope.
As far as the driveshaft bushings are concerned. I think I was misreading the torque tube. More exactly, I had unbolted the ball joint flange and the loose torque tube itself was flopping around on the driveshaft when I spun the rear tire with my foot. I do not think my main problem is the bushings.
The vibration is always there at higher rpms when the engine is in gear - at 25-30 mph in second gear and above 45 in third gear. The 235 has plenty of pep though
