For me (and again, it sounds like our symptoms are somewhat different since your neck seems messed up), it's like I feel a pressure sensation in my head opposite to the direction of movement. When I used to walk "heavy on my heels", as you describe, the pressure sensation would be on top of my head, and if it was great enough, I would feel it as more of a pain than a pressure. If it was greater still, the pain would linger after the motion had stopped (as a headache). Same with bumps in the car. But stopping quickly in the car, I'd get it in my forehead, if that makes sense. Took me a while to pick up on the pattern of it, but the pain/pressure was always in the same "plane" that I was moving in. Once I learned that I was just feeling sensations that I was supposed to feel but WAY amplified, it all made a lot more sense.

These "headaches" never came with any other symptoms and never lasted any longer than a day or two at the worst (though the symptom itself was disturbing and did cause me anxiety until I figured out what it was and adapted to having it).

For me, there were five or so instances where I moved significantly more suddenly compared to what was "comfortable" at the time (tripping and falling, for example), and each time it cause me two days worth of really awful additional symptoms (fogginess, dizziness, head pressure, ear popping, etc.) and then about a 6-month regression in the motion sensitivity symptom. These were what was diagnosed as "migraine attacks" (with primarily vestibular symptoms).

I don't know if any of this was helpful..,everyone is going to be slightly different, and I never had the neck issues. You will find so many stories of it on here, though! Hopefully you can find a good doctor. I honestly went through about 6 or 7 before I found one who really knew his stuff, and it made a world of difference in my progress, so don't give up on that. Don't be afraid to advocate for yourself and ditch one if they aren't working.