Drivetrain wobbles??

[FastMaxx]

I'm having a weird problem with my K5. Its an 86 with a 350/sm465/np208. 3/4 tons with 4.11 gears and 38" tires.
It seems the entire drivetrain wobbles side to side. It only does it while cruising in 4th gear (50-55 mph). I have replaced the trans mount and even installed Kurt's poly engine mounts. They helped a lot but the wobble it still there, just a tighter wobble. Any ideas?

 

[Jester1]

Had a problem like this once many moons ago. Turned out to be a u joint that would throw the drive shaft out of balance. Can't remember which one but it was on a dodge van.

 

[diesel4me]

A u-joint with one cap full of rusted needles can cause a binding action that can cause that,without making any of the usual squeaking noises assosiated with a bad one..till it gets really bad anyway...

Sometimes a u-joint gets bound up by the yoke ears if the shaft got whanged in use or while changing a u-joint...shaft itself could be bent too,or the yoke may have gotten tweaked somehow..also make sure the slip yokes splines are not sloppy and the yokes are properly phased 90 degrees apart too,that can cause all kinds of troubles like this...