Easy stuff first - have you changed the tires lately? unbalanced wheels or even some tires (my OE ones did this) can cause this, have a look for a clean spot on your wheel rim where a weight might have fallen off also.
As said by others addition of luggage can cause it or saggy rear sussies (ooer missis).
If non of these, then wheel bearing, head bearings, twisted forks, etc, etc, but tink about thye simple one first..