Guess what ?
My van recently started with a similar problem. Steering wheel shaking - upto an inch of movement - at speeds upto about 35mph, after that it calms down until barely noticable.
Suspecting a wheel bearing or driveshaft, and having 3 inches of solid ice on my driveway, I took it to a friends vehicle testing station.
Put it up on the hoist, wheels off the ground, and he inspected everything but found nothing at all wrong mechanically.
Then he started spinning the wheels. Even with 5mm of tread on it, the left side front tyre is history !
There is no visible damage at all, but when he placed a bar across the tread and spun the wheel you could see that the tyre is no longer round !
Tomorrow I get 2 new tyres for the front.
P.S. If you think a driveshaft is worn, drive down a quiet piece of road and slip the transmission in to neutral. If the van is still shaking, it's unlikely to be the driveshaft.