My Ford F-150 has a shudder at 45 to 50 MPH under a light load. I think it could be an ignition coil or spark plug problem.
A shudder could be a problem with an ignition coil pack. On most cars they start to fail around 150,000 miles. I would have a technician come out and inspect the coils and you will probably get a p0300 code as well if it is misfiring. It could also come from a bad injector as well. A technician will be able to pull the codes to determine where to start looking for your problem.