I thought I had a similiar problem with my '94 4X4, but when I pulled apart the dash and pulled out the instrument cluster I was suprised to see no speedo cable connected to speedometer. Maybe its because its a 1994, but there was no speedo cable must be electronic sensor I guess.
My speedo needle didn't shake, but I can hear a ticking noise when driving and its not the engine (ticking doesn't follow RPM's of engine)

I am missing somehting ?