I think the ujoint was worn and the yokes were stretched out from earlier wheeling adventures.
Our trucks have a way of finding the "weak link".
Do you have a line on the axle shafts yet?
Maybe try DTS, if i remember way...back then they actually sold an "improved" version of axle shafts. "Improved" meaning possibly a harder grade of steel and shafts they were a consistent thickness, (where the diameter was not turned down over the length of the shaft).
Some parts have actually evolved into a better quality over time, (unlike most cheaper repopped parts).
Jeffviewtopic.php?f=22&t=46251http://s240.photobucket.com/albums/ff13 ... t%20build/
71 F-350 dually flatbed, "little 302" (as some folks here call them) .030 over V-8 with a "baby"C-6, B & M "ghetto" truckshifter, Dana70/4.11 ratio, intermittent wipers, tilt steering, full LED lighting on the flat bed, and no stereo yet (this way I can hear all the rattles to diagnose them)!
A lot of "oddball" Ford parts collected from working on them for 34 years now!