UPDATE:
Boat was finished yesterday, so I took 1/2 day off work and relaunched this morning....
Got the boat into the water, warm it up, untie from the pilings and go to my well at the marina and NOPE - gearshifter will NOT move either direction... I can press the override button on the throttle and gas the engine, but obviously the shift cable is bound up somewhere
Called the mechanic, he came out and in the water, adjusted my shift cable. He said I need a new cable and its his fault - he will replace it. He adjusted it so I can go into forward easily but reverse is a real bear. It takes ALOT of muscle to get the shifter into reverse. Its definately bound up somewhere...
With that being said, I took the boat out for a 20 minute spin, MUCH better.
Parts the mechanic replaced after the faulty u-joint installation he performed:
Gimbal housing, gimbal bearing, u-joint assembly (The whole complete shaft this time!), bellhousing, bellows. I have all of the old parts and MAN are they torn up..
It was a complete waste as he just replaced most of those parts (gimbal bearing, bellows, u-joints) in January 08. He took blame for blowing up u-joint and the bad shift cable now.
Anyways, the boat rode very nice, the smoothest its ever been. Im kind of concerned when Im on plane and turn, it makes a very slight subtle rumbling. So fast forward, I ended up launching my friends boat (268 Vista) with the same powerplant and his rumbles too like mine does. It doesnt do it in neutral, so it has to be the nature of the duo-prop beast.

Im having the same issue!!! I just replaced everything, new gimbal, shaft, u-joints, crush sleeve, engine aligned. I have a vibration on turns. this normal for what your saying you have?