The usual complaint, which I have seemingly just cured, is noisy vibrating in 5th and 6th, also sometimes, accompanying this, is a low harsh jangley rumble at low speed, like tickover in 2nd.
It seems to me that the propshaft angle has a great deal to do with this, by "wiggling" the loose flange on the shaft, it must be just the prop speed that stops it happening in the lower gears on the road, as the resonance and harmonics change with the revs.
For now, you could try the low speed test for coarse rumbling, if it does it tighten the flange and see if it goes away, just a thought.......