Help ID that clunk... 1970 edition

[GeorgeF]

I am having trouble identifying the source of a suspension or driveline clunk I experience in my 1970. It is a soft impact that hits within an instant of stepping on the gas and also when I am braking, at the last second before the rear suspension re-loads after stopping.

- There is no accompanying vibration, so I have ruled out a worn driveshaft ujoint or pinion/output bearings.
- There is no hard/sharp metallic sound, its just a thump, kinda like someone hit the tub from underneath my seat with a rubber mallet.

I suspected the rear leaf spring bushings had slop that was allowing the rear axle to shift back and forth, but I can't find anything obvious. As far as I know the spring bushings are all OEM still. What else should I check?

It has 2" lift blocks that the PO put on, and they aren't moving around.... ? Thanks in advance for suggestions.