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).

Jeff
http://www.fordification.com/forum/view ... 22&t=46251
SOLD-71 F-350 dually flatbed, 302 / .030 over V-8 with a "baby"C-6, B & M 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 the rattles to diagnose)! SOLD!
Many Ford bumps / one 76' EB / and several dents through the years.
A lot of "oddball" Ford parts collected from working on them for 34 years now!
2008 Ford Escape 4 x 4