hey guys! i just had a similar prob, i had a gearbox replaced and the car had vibration
at over 80kph, it ended being the centre bearing was shit and the tailshaft needed to be 15mm longer. but the damage also went to the yoke seal on the gearbox.
best thing to do is take it to a driveline specialist. not including the price of the box,
that monkey mechanic who didn't check his work has cost me $1200.
could have bought a rb25det box after all,,