Oi! Wayne! Vibes!

Pinion nut? Might be your issue too!

Drained the diff oil, clean as a whistle. No howling on the over-run.
My cyclical vibe operates at approx. 1 second tempo, varies with speed.
Felt predominately in 3rd and 4th gear, between 50 & 70kph.
In 1st & 2nd, it seems rpm is too high in the 50-70kph speed range, and loadings are insufficient at slower speeds to cause any issue.
I don't normally use 5th & 6th in the 50-70kph speed range, I have tried it but can't simulate the symptoms. At any speed in the top 2 gears it appears free of any issues.

Tell me more, what should I be looking for?

Maybe its the big handle bar nut, not the pillion nut!

This happened since you changed the tank? if so try the old tank again.