Try the same 15 mph in a couple of different gears. The motor has enough torque to pull 3'rd on a flat level parking lot at a steady 15 mph (I think) If it will, and engine gyroscopic forces are to blame then the effect would be much much less at lower RPM. If it is unchanged, (and I would try shifting to neutral and coasting once also) then it is alignment for sure. One other thought, is there any unusual tread wear? If the bike spent the last 4,000 miles on an oval or commuting in the slow lane of a road with a very high crown,... OK I'm reaching a little here, but do check the tires.
Martin