PLEASE HELP.. VIBRATING DECK!?

You never stated if you ran the mower WITHOUT the cutting deck ..???
If so what were the results ?? By doing so you determine if the problem is in the deck or in the tractor ...

"Runout" The shaft should turn with no wobble...like a pencil does in your fingers. Any small "runout" is magnified to anything attached to it....the star holding the blade. So the blade of slightly bend spindle shaft (hit something?), or a worn or loose bearing will transmit imbalance from the rpm of the unbalance blade through the deck. This is what happens when you run an unbalanced car wheel. A little problem becomes pounds of force felt through the floor and suspension. You need a way of seeing if the shaft is "true." The wire pointer is poor man's dial indicator. "Dial indicator"...a little gage with a contact point and a pointer showing movement of the shaft, if any.

Also check that the 4 bolts that hold the engine in the mower are tight.
An please answer the previous question so we can work out if it is in the deck or the mower.