Thanks Klaus.
The use of C.V. joints on a propshaft is a fairly modern practice, and may not be widely understood by precedent.

It could well be that the higher speed,(engine r.p.m. in top gear)causes the lube. to "cake up" centrifugally when it ages, and with little angular displacement (unlike front axle C.V. joints) it stay were it was flung,inside the gaiter (or boot). I notice that propshaft gaiters are not convoluted as axle ones are, this may be an acknowledgement of the higher speed by the designer.

The mixture of C.V.joints with a none C.V. joint is intersting and probably quite acceptable if the angular displacement of the non C.V.joint (or universal joint if you like) is absent or small. However, with age, wear, stiffness, slackness, friction, lubrication, torsional vibration, road surface, and the moon and stars being right;it may start a "chatter" in the slightly worn C.V. joints.

"Bumping", is I believe, caused by the relationship between the weak positional register of the centre bearing mount, and the dynamic stiffness of the under lubricated C.V. joints when under the loads imposed by the difference in the speeds of the front and rear axles, on tight turns. Just observe the differing radii of the wheel tracks of a car on snow when making a tight turn.
I believe that the slight stiffness in the C.V. joints will translate into shaft misalignment that cannot be resisted or resolved by the floppy centre bearing mount, and this results in vibration/"bumping".
The viscous coupling cannot help if these forces don't reach it!!
Best regards, Howard.

Self preservation is the last law of averages.
(Stan Laurel)