I'm not sure what you mean by "shudder bucking" but I have a serious vibration problem with my 2017 Buick Envision (Essence 2WD 4 Cylinders A 2.5L, 68k miles). It happens when driving at highway speeds and only occurs randomly/very occasionally. I get a very strong vibration where the entire vehicle shakes for 5-10 seconds and then stops. I took it to my mechanic and the dealership and neither could do anything since the issue is not reproducible (and there's no "check engine" or any other lights). I haven't been able to track exactly what I was doing when the problem occurs, but this morning it happened when I was in cruise control at ~70 MPH executing a very small turn while hitting a bump on the highway. I tried getting the tires re-balanced and rotated but that didn't help. It's very concerning and seems dangerous. My wife will not drive the car now. I'm very frustrated and do not know what to do.