Do you have lesions in your c-spine from your MS? I do, and have had crazy sensations (up and down my back), lhermittes sign they call it most oftentimes.
Mine was when I would look down, so the bending of the neck and damage within the cord at those levels is why I would experience the vibrating, but mine went directly to my keister - crazy. Soon as I lift my head, gone!
Did this just begin for you?
Asking because looking at your original post, appears you've had MS for years, and I'm wondering if after all these years you possible could be experiencing some new inflammation, or you tweaked something in your neck.
I no longer have lhermittes, and I feel it's because I'm past my inflammatory phase of MS.
It's important to speak with the doctor to find out for sure what's going on, but if it were me, I'd take note to the position I was in when the vibrating is happening and see if an any neck adjustment stops it, or increases it.
Thankfully that sensation is mostly just bothersome, and not painful or limiting.
Hope this helps : )
We use to have a healthpage on this, but I cannot find a post of it. But, this is an excellent article about lhermittes.