van is fine under 45 mph. Once over 45, it starts to “bump” – rough ride like something is wrong with a tire or something. (tires are fine.)
Is it the torque converter?
If your car shakes and shudders, it could mean the lockup clutch is malfunctioning. This might occur at speeds around 35-45 miles per hour. It’s a noticeable problem and feels like you’re driving on a road with lots of small bumps. A worn out lockup clutch can make the transition from acceleration to direct drive difficult, and it’s a sign that you need to have your transmission checked.
Source: https://aamcominnesota.com/reasons-why-your-ride-is-bumpy-and-rough/