Allowable backlash

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.

If the spider and side gears (and case) are worn badly enough, that could certainly cause it.