Can't locate my violent vibration problem- Help!
 

Hi all,

I have a 98 Chevrolet Blazer 4x4 (although I only use the 2WD) ~170K miles. I have been trying to isolate the location of my vibration problem with no luck. I'm hoping y'all can help.
The vibration is primarily experienced when driving on the highway around 65-70mph only when I let off the gas ( 2 second tight violent vibration and then completely goes away as the truck decelerates itself) or briefly as I accelerate. I thought I felt it underneath the drivers seat, but my nephew said it felt like it's coming from the back. Who knows...
Anyways, I took it to a shop and they told me it was due to all sorts of suspension parts. I doubt they actually drove it... so i replaced them : front CV axles, front upper control arms, sway bar links, pitman arm, Front drive shaft U-joint, rear drive shaft both front and back U-joint.

The problem still persists and I have to drive super careful (60-65 and have to maintain a constant speed) as to not cause the vibration because it feels awful and I'm sacred It is wearing something down. Recently when I was driving back home, I received an error code for slipping transmission (P1870- I'm guessing TCC valve since it drives fine around the city cold and even on the highway when I'm not driving 70-75) but this might be unrelated.

Thoughts?

Thank you!
Peter R