Hi , I have a troopy 78 , with 50 mm lift on it , never ever had any issue with vibration or worn out U joints .
260K 80% of it off road.
Castor correction will give you the right weight / angle on steering ,should not affect vibration.
If you have a vibration problem , address that issue first .
Someone has highlited that the new prop shaft is out of phase , 100% true .
Can't work for long like that .
In my opinion no need for a double cardan shaft .might get vibrations worse.
More parts , more moving joints , more tolerance at play.
Ever thought to change mechanic ?