91 Cummins shake

I have a 91 w350 4x4 dually 5 sp.  The truck goes into a shake and you have to come to a complete stop to fix it.  Various speeds and road conditions.  Doesn't do it all the time.  I have replaced all ball joints all the steering components including gear box, and wheel bearings.   All new rims and tires, transmission mounts, drive shaft rebalanced.  I'm at a complete loss.  What am I missing?  No frame cracks, stock wheels and tires, no mods except for a flat bed.  Rear axles not bent, checked those.  Can someone please help me out with some ideas.  Thankyou!  

Double check your u-joints. Sometimes they could come apart if not installed correctly. How about your motor mounts are tight? Hows your clutch? Bent rim? Poor balance job on the tires and the weights fell off?

Just some suggestions. I've been in your shoes. ended up being a shitty tire that wouldn't balance. Than i would get it only in 4x4 it was a u- joint.. I thought they were good because i just installed them obviously i did a poor job.lol

I hate it when my truck shakes... I hope you figure it out bud!