My sons 1999 Ram vibrates/shakes like crazy when it shifts into 3-4 gear at around 45 mph and higher. When the rpm goes below 1500 it starts to shake but if you accelerate and down shift with higher rpm the shake mostly goes away. Its almost like a real bad miss in the engine. If you take your foot off the accelerator while above approx. 45 mph the shaking completly goes away. Both front bearings and tie rod ends have been replaced...ball joints are next but I don't think that has anything to do with it. The motor and trans. mounts are good. Could it be the torque converter? . Please advise..

 

https://dodgeforum.com/forum/2nd-gen...um-thread.html

Go through that link.

Often times the #8 spark plug will start to misfire. Pull your #8 spark plug (last one on passenger side) and see if it's covered in gunk/oil.

I had the same issue and couldn't get past 45MPH because of the heavy shaking. It's also been advised that could be a clogged cat too, anyways...

Read the link, check your spark plug. Go from there.

 

That is a misfire. Mine did the same thing. Felt like the truck was going to shake itself apart when it hit 4th...... Check the plugs and tuneup parts, and check your plenum.