Proctor750

My 86 2.5 diesel 5 speed likes to jerk violantly back and forth the most in low 3rd gear. Usually after a gear change from 2nd to 3rd. The colder it is the worse it is. The longer you drive it the more manageable it is. But no matter how I finesse the accelerator pedal, trying to calm it only makes it worse without letting off completely, or downshifting and getting back into 3rd at higher rpms.
It feels like the more low end torque applied, the worse the effect.
I have read several threads with no conclusive evidence of the cause. I have checked both guibo's (flex discs), center bearing, and so on. There really is not any play and they appear new.
Another thread mentioned something about a butterfly valve? I don't know if they are talking about the throttle body valve or what, but I can't imagine this could be the cause.
The only lasting idea I fear may be the culprit is someone replacing their Diff with a junkyard diff and the problem going away....
I can't imagine any amount of back lash that could cause this much jerking in the driveline.
Any Ideas?

Proctor750

Also I have read a few posts about the "shock absorber" on the throttle linkage, and something about the injection pump. Both seemed a bit ridiculous.