Sorry there haven't been more timely replies for you. There is a number of things this could be. Worn steering track rod ends and worn or damaged driveshafts or other drivetrain components come to my mind first. It could also be to do with power steering. If you've had wheel balance checked since wobble started, then it's unlikely to be a damaged or buckled wheel, or a damaged / unevenly worn tyre.
Without a lot more information it would be difficult to diagnose here.
That's why Ahto42 suggested getting a mechanic to investigate closer.
Did you get a resolution in the meantime?