Allowable backlash

Backlash usually does not show up as a clunk. That's normally slop in the pinion "somehow" OR bad U-joints.
Rusty and others
I installed a new driveshaft, tranny yoke and replaced both u joints last year, it made no difference in the "clunk" The old shaft was bent and the slip yoke was junk from 50 years of use.
At this point I am leaning toward the spider gears and shafts.
I will learn more and report back next week when we tear into it.