I, too, have a hard time narrowing down why flying makes my PN pain--and numbness--worse, but I'm certain that vibration has a role to play. Even when I've taken a relatively stress-free Amtrak ride from Portland to Seattle (with very little elevation change in-between), I'll notice it.

Part of this seems to be irritation of my lower back--when my spinal health is good, all my neuropathy symptoms decrease. And when I'm being vibrated by transportation, I think it irritates my spine--and that's where the other end of my nerves are, after all. Shrug.