[commodor47]

A few more ares to check come to mind;

Do you know if the previous owner had similar issues?

Check the frame alignment.

Check the roundness of the wheels.

Check the axle for alignment with the coupler.

Check the axle for damage due to being used as a jack point.

Your latest post seems to indicate a vibration issue while under tow. Some of the above could be the cause of what you are seeing as an alignment issue.

Dick