Hey everyone,
I have an UPDATE!! I finally managed to fix the problem.
This week i bought from a local parts shop a driveshaft, the left-hand side one(gearbox side), fully complete and refurbished, so i went ahead and changed it this weekend.
The old one had a tear in the boot (gearbox side) and went like this for a long time, months really...and did many miles with no grease..very thoughtful from me isn't it?
It was in a very bad state. The spider was completely shot and some of the rollers were nowhere to be seen, only the ball cases remained in place luckily, but were very loose.
SO after a test drive to see if everything was assembled correctly, it all seemed fine and the vibration(shutter/shakiness) was nowhere to be seen.
Only thing left is to change all the clutch assembly, including the flywheel. It is already on the way and i'm one step closer to a clutch-slipping free life

Take care,
Rui louro