I'm picking up bad vibrations

[Fergus McVey <Fergus.McVey@domain.elided>]

My 2000cc Bertone Coupe is vibrating quite a lot at higher speeds. It seems
to be coming from the drivetrain, possibly the diff. The symptoms are: At
around 40 a metallic rattling sound starts to build, at nearer 70 it is
accompanied by plenty of vibration, getting toward 80, 90 it seems to be
evening out but is not gone. I didn't dare crank it up any further. When I
ease off from 80-90 I get a peak of vibration at 70ish which causes the gear
lever to resonate, then it gradually subsides. When I put my foot on the
clutch at these speeds the vibration increases dramatically.
So, mindful of the fact that one can't expect to get a diagnosis here, are
there any tricks to help me diagnose what it might be? I reckon it might be
an unbalanced prop shaft, or the end bearings in the 'box, does anyone know
how I might work it out? ALso I've heard that these diffs are pretty much
bullet proof, or is that cobblers?
I should point out that the 2ltr was fitted quite recently and that it was a
1300 before, it is supposed to have an LSD from which I'm getting a little
'clonk' as I pull off. I've just had a complete new set of tyres fitted and
balanced, but the vibration was there before.
 
Heres hoping
 
Fergus
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