As I live and breathe, D, I've been there and beyond. For my case, it is definitely spasticity and actually starts in my back. It's like an involuntary and unrelenting stretch like with a yawn. It hurts and it's irritating just as you said. I call it the Frankenstein walk.

The exact same conditions, such as major stiffness after being in the car, worse at night, and loosening up when I start to move some are present in my case.

If you want to see something freaky but kind of cool, when you first get out of the car, let the leg spaz have its way by hanging your leg loose just off the floor— if it's like mine, for a few seconds, it will move faster than any human should ever be able to move. It's just so weird but sort of funny.

Ask the neuro if you can up the Zanaflex, especially at bedtime and in the morning. Hope that helps. MMJ works very well for it, but strictly at bedtime for me.