I'm guessing it will be ok with the chain off, as the load will be removed from the over/under. Could be the pilot bearing that centers the output shaft to the rear of the input shaft.

How early of a 1755 is it? Those early over/under units had a direct drive clutch drum that used only 2 retaining rings instead of the three used later on. This can allow that drum to wobble on the output shaft, causing vibration.

Could be as simple as my 1655 was. The splines on the output shaft and the sprocket were both worn, and the nut had come loose, causing a vibration. It was different in each speed, but under was the best in my case. I couldn't find anything else wrong with the unit.