Vehicle shakes when accelerating through certain speeds

My wife's van is having a difficult time maintaining a certain speed at certain points when accelerating, particularly when going uphill. It used to happen between 40-45 mph, and is now around 55 mph. At this point, if you don't power through (and thus possibly speed) those points, the car will shake like it's not sure whether it wants to accelerate or stall. As a result, it vibrates quite a bit. Decelerating causes no issues, nor (typically) does flat ground, or other speed points.

Any thoughts?