Interesting Graham - I had a constant and quite severe shake on my -99 4/4 when I picked it up outside London a few years ago. Had to stay under 60mph all the way to northern Sweden...
Finally it turned out to be the tyres. But it got me thinking - I could feel that the whole front end was flexing and that could eventually cause a crack in the frame if there is a weak spot somewhere. My frame has the triangular reinforcements and I have full confident in it still
I'll stick to this one (1) for now