Originally Posted by scENFORCER

If its a constant vibration that progressivly gets worse the faster you go, then its something to do with your wheels (likely balance, but as Ashtray showed us it could be a few other things like a brake caliper).
But if the vibration is only at certain speeds (ie, 40-45 as OP stated) then its likely the transmission mount. I had this same issue in my previous '99 and swapping out the tranny mount cured ALL vibration issues I had. It was a stupid easy job to do, and worth doing on ANY sc over 80k regardless of whether you think you have vibrations or not. Once changed, you car will run even smoother than you thought it did before

..same as the tranny mount, the engine mounts are made the exact same way, so if your tranny mount needs replacing, then likely your engine mounts do too.