I have a 95 Dodge Ram 1500 pick up that shakes real bad mostly in o/d at 40+ mph when I let off accelerator it smooths out. I down shift and it gets pretty smooth, not always completely. Can't figure this one out I just know its not in steering wheel.
It sounds like it could be a bad drive shaft (universal joint/velocity joint).
Removing the drive shaft will help you determine if it is the front or rear joint.
When you remove the shaft, from the truck, one of the joints will be stiff and will hardly be able to move.