Mystery vibration at 25 mph can't be diagnosed. It's not the front end!

I worked at a CPD dealer in 1986, the Lead Tech told Me to pull in this gold 318 4WD tk. & pull the rear end cover. I did, then asked what we were looking for. 4th time in
& had a shake/vibration/shudder at speed, everything in the diff. looked perfect, clean and tight. Buttoned it up & checked for missing drum weights, etc. The Service Mgr.
told Me to take it for a ride, at certain throttle positions 40-50mph it would start bumping then bucking, at one point it shook so bad it made it hard to focus thru My specs!
The rotor rivet holding the tip was loose & it would change position at a point where the advance under certain eng. loads would let it jump to the adjacent post...........
This "shudder" could be a bunch of things, certainly prop shaft issues as well, things bind under load that seem fine in Your hands. Lesson, assume nothing & check
everything..............