Several things that can go wrong with the propshaft. If the centre bearing has ever been renewed its possible the flexible mounting might be slightly misaligned where it attaches to the body. The prop shaft has to be split to renew the bearing but its important its reassembled in the same way as it was found as AFAIK its balanced as an entire assembly. The propshaft itself often has balance weights welded to the shaft- sometimes these can come off. The doughnuts can have bolts with either balance nuts 1.5,3,5 gms for M10 bolts or 8,10,12 gms washers for M12 bolts that can be added to suppress vibration. Not saying this is your problem but vague but discernable symptoms are often down to incorrect assembly or substitution of genuine parts and while perfectly safe may display annoying symptoms like this.
One example of this I had many years ago was a Macpherson strut car pulling constantly slightly to the left. Tracking was checked--- it was OK The mystery was finally resolved by loosing the strut top mounting bolts with the car's weight sitting on its wheels and allowed to settle. Bolts retorqued ----- tracking perfect.
