when we got the Vit6 in 2003 the prop slide was absolutley awfull, slack , wobbly worn , pretty amazing as it doesnt move much, maybe thats the culprit
for a while we glued it up with heavy setting silcone, but like yours soon reverted to a new replacement,
we had vibration but this did not solve it, after chasing it around the car with bent engine back plates , clutch disc damper springs, a replacement ratio diff, the solution was the inner UJ cups having small amounts of float, new UJ with thicker Circlips solved a lot of head scratching.