A clunk? Is that the sound a chicken makes while at the junk yard?
Sort of a cross betwwen cluck and junk. :)
U joints tend to make a clicking sound when you let off the gas.
It may be just starting to go bad and is just "clunking"
Also check the rear springs for a broken leaf. That will make a clunk.
On the speedo drive, yes the nylon piece does wear out or break.
I have replaced a few to fix a non functioning speedo.
Very simple, just unscrew it and replace it with a known good one.
-- 
Frank Clarici
Toms River, NJ
5 British cars on the road
http://www.exit109.com/~spritenut
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