You cant break stainless steel cutter blade, welded parts and steel ball bearings with water mellon rinds.
Even a metal fork should not break a garbage disposal ... the fork will wedge against the side wall and cause cutter blade to get stuck. Once the fork is removed, no damage is done.
If insinkerator is shaking, then either a bearing has rusted out, or the shaft has finally worn out.
The stainless cutter blade is permanently attached to a steel shaft that goes into the sealed bearing. If the cutter weld is broken, then the disposal will wobble. If the bearing wears out, then the steel shaft will wobble. If the steel shaft is worn down at the bearing, then the cutter plate will wobble. A tiny bit of wobble will cause the disposal to shake. A brand new disposal will vibrate.
Check date of purchase receipt.
Amazon shows warranty is 2 years:http://www.amazon.com/InSinkErator-Badger-Food-Waste-Disposer/dp/B00004U9JO/ref=sr_1_3?ie=UTF8&qid=1303015273&sr=8-3
If disposal is 3-5 years old then that is a short lifetime for a solid product like the 1/2 HP.
If disposal is 5-8 years old, that's a good run. 8-12 years is very good.
I wrote tips for replacing disposal from days when I worked apartment maintenance:
We replaced disposals every 6-7 years. Some never wore out. Others went bad right away.http://www.fixya.com/support/r7929008-replace_garbage_disposal