I've had the same issue with my van for years and no mechanic managed to fix it. The van has gone around 230,000 miles and it shudders at the speeds over 50 mph. The shaking that comes and goes in the intervals of about half a mile, can be felt in the entire van. The steering wheel has no torque. This shuddering is very similar to an unbalanced wheel and is very bad when I reach the speed of 70 mph. I therefore don't drive that fast. The tires have been balanced. I had some repairs done but it didn't help with the shuddering. Do you have any idea how it might get fixed?
As you speed up there is normal tire gyrations that the shocks and suspension will dampen out. When the shocks or another steering component go bad, it no longer can dampen out the vibrations. The shocks and king pins should be checked to see if they need to be replaced. If you would like, a certified technician from YourMechanic can come to your home or office to inspect the shakiness coming from your vehicle and make the necessary repairs.