I see what you mean about the pucks. Looks like a 240Z? Later ones were scalloped.
If there's a gap and they're hard you can get a clunk when the diff and mustache bar twist under acceleration or deceleration. Or just from riding over bumpy roads. I haven't seen separate aftermarket parts, kind of surprising. Seems easy to reproduce. Here's some part numbers if you want to look around for NOS. Any year will work, the difference is the shape.
30 and 31 -
https://www.carpartsmanual.com/datsun/Z-1969-1978/axle/rear-suspension
The shifting clunk usually comes from the diff nose rising up then dropping back down, because of a loose front mount.
The basic problem, that Nissan also was trying to solve, is that if things are too tight you get diff noise/whine transmitted in to the body. If they're too loose you get thumping and clunking. It's a balancing procedure. Probably best to ease your way to where you want to be.