Odd vibration, pulling

[Jim Wright]

This evening on the drive home, I started having a bad shimmy and my
Classic was pulling to the right.  I stopped, thinking I had a flat,
but there was no obvious problem.  I started moving again, and the
vibration didn't start till about 30mph, and got very bad when I hit
40.  Letting off the gas stopped it for the most part.

But, after about 2 miles, almost to my house driving under 30 the rest
of the way, suddenly everything cleared up!  So now I'm totally baffled
about what might have happened.

My 2nd thought (after a flat tire) was that the passenger side axle was
shot, but obviously that wouldn't have gone away like that.  Next
thought was some form of debris lodged between the brake pad and the
rotor, or something along those lines, but I have no idea at this
point.

Anyone have any thoughts, or recognize the symptoms?