Thanks for your suggestions, guys, but I've got all those things covered, (I think...)
As I said, all the U-joints are in good shape, All the suspension bushes are new- this car has been completely restored.
You can bounce up and down on it all you like and it makes no noise from the suspension.
I used Kendall posi additive in the oil this last time, but when first assembled, I used the GM stuff ( a little hard to get here sometimes). The posi clutches do make some noise on turns, but what I'm experiencing is a worse clunk which happens when driving dead straight, which kind of rules out posi problems, I would have thought.
The clunk has a kind of 'snatching' quality to it. It certainly seems to cause the wheels to judder, and seems to be mostly coming from the passenger side- I sat in the bed and listened while the missus drove it.