Vibrations during sea trial

Have a driveline vibration issue on Watermark--53 foot Huckins Atlantic.
Starboard side no issue.
Port side while underway you can visually observe drive line start to jump around increasing as you increase RPMs
Have hauled the boat to try and solve problem.
Watermark was repowered by Huckins in 2001.
Only 800 total hourson Commins C450s which are running smoothly.
Props were replaced 100 hours ago and visual inspection looks ok.
The driveline is a Python unit with CV joints on each end.
First step we pulled the Python driveline and reversed it!
This actually reduced the vibration but not to an acceptable level.
What I have been told is you can not get replacement parts for the Python units????
The VEE drives are Twin Disc units again only 800 hours.with the boat out the water properly blocked nothing seems to be loose or can be moved around by hand!
Trying to figure out next steps to take.
Any ideas how to proceed?

Could be U joints, could be even internal in the engine.

Thanks for your reply Capt J.
The Python drive line coupling does not use u joints per se.
Each end of the drive line has a CV joint and yes they are suspect and the plan is to remove the drive line and exam the CV joints!
We have considered the engine could be the source of the problem.

The 1000 hour service was just completed and oil analysis done!
Oil came back all normal and there aren't any other obvious signs it's the engine or engine mounts!
Boat does have Glendening synchronized and they have been checked and appear to be doing there job

I once ran a 58' MY with 8v183's, it was a low hours boat (around 600 hrs) but about 10 years old....when my guy who took it in on trade survey/seatrialed it, the port engine grenaded. The seller paid to have the dealer major that engine which they did. The boat ran very good by all means. BUT, it had a vibration at planing speeds......noticable......props checked perfect, u joints looked perfect, alignment I had checked but nobody could figure it out. About 300 hours later it snapped 2 metal injector lines on that motor which I suspect from the vibration, we replaced those. I suspect it had a slightly bent rod or something else in the rotational mass of the engine.

Sorry to hear about that ugly experience!!
I have not dismantled my Python drive at this stage BUT think it is next step and since thus far we have not found anyone who has replacement parts I'm curious did you get your Python drives back from the shop you sent them out to for repair---not likely given I don't know the size of your drives but there is the possibility you have some salvage parts?
Right I'm an optimist
Thanks for responding--when did you have this misadventure-- last year or so???

Forgot to ask---was there a reason you didn't just switch to Aqua drive units which I have been told perform a similar function to the Python units?

Thanks for the details!
Will research Centaflex.