Excessive engine vibration is a telltale sign that the balance shafts (also known as silent shafts) are out of alignment. These shafts are designed to counterbalance the engine to keep it from shaking during normal operation.
If the engine vibration just started, do NOT start or drive your car until you can verify that the balance shaft belt is okay. If the belt is old or worn, it can jump, causing the balance shafts to be out of phase. This, in itself, will not damage your engine, but is a symptom of much larger potential problems.
The real problem is that if the balance belt jumped, it may be getting ready to break, and could the next time you start your car. This in itself is not a bad thing (the car runs fine without the balance belt at all), but the balance belt has a nasty habit of hitting the timing belt after breaking. The timing belt will often jump or break after such treatment, which is, literally, an engine-destroying event. You can count on losing at least eight valves if the timing belt jumps, and probably all sixteen if it breaks. Repair costs can run into the thousands. It is for this reason that the balance shaft belt should be replaced at least as often as the main timing belt.
It is also possible that the engine will shake immediately after a timing belt change. This is indicative of a simple misalignment of the balance shafts in the engine. Running the car that way is not damaging, but is obviously undesireable. Return the car to the shop in question to have the timing re-done - driving it there is usually ok, although towing is great (especially if you can get the shop to do it). Read the timing belt VFAQ for more information. (Yes, it's listed in the FAQ Locator.)
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