Bad vibration, rudder has play

Hi all,

I developed bad play. It seems like it came out of no where. We had been out for a couple of hours and then all of a sudden terrible vibration. I didn't hit anything. No damage at all.

Here is what I have done so far.

Prop, is ding free and balances perfectly.
Had the prop shaft aligned.
Had the shaft checked to make sure it's straight.
The strut bearing had a tiny bit of play so I had it replaced.
The tranny and engine sound and run perfect!

Here is what the mechanic said. The rudder has about a 1/2 of play in it. I can shake it back and forth pretty far. He said it shouldn't move at all. I am already 700+ into this and I would like to know if anyone has had this issue. I think underway that 1/2 of play flutters back and forth causing my vibration?

2003 Ski Centurion Avalanche c4 with 700+ hours on it.