Hi Boule de feu
I'm not sure if we're talking about the same thing or not but I used to get a feeling of vibrating, or an inner tremor -- had it for years. Sometimes I felt like I would shake apart, even though to the observing eye, I was not shaking. It would often be accompanied with a numbness in hands, lower arms and face. I had it pretty much non-stop for years.
I've been slowly recovering (3 years and still not all the way there yet) and even during most of this better time period I have experienced the inner vibration. It's gone alot of the time now.
I'd say it's definitely a CFIDS thing, in my case at least.
Guesses as to cause -- messed up nervous system messaging -- like sending/ receiving static -- the equivalent (sort of) of vertigo in the ears, only this can be a widespread body experience. Other guesses -- messed up adrenaline and cortisol levels -- too much of one, not enough of the other, wrong one having to be used at the wrong times ... take your pick.
Another guess. Deep, profound, beyond the pale, exhaustion.
I don't know which of these -- if any -- are factors. Could be something else completely. I would take it as an indication that more rest, more often, is needed. I would look into any possible food sensitivities that might be making things worse -- when I went low carb, some of this stuff (not all of it) disappeared. So, low carb doesn't cure it for me but it keeps it from getting worse from eating stuff that my system can't handle.
All I can think of for now. Try not to let it get you too freaked out. As freaky as it is, it can be dealt with. As far as I know it's not dangerous. And it can go away, even if you've had a severe long bout with it as I did.