1998 Dodge Ram Truck Shaking

I have a 98 Dodge Ram that runs good My husband changed the U-joints, Tie rods ,drive shaft center support did a Tune up, and the Truck still shakes when we hit 45 mph it will start shaking alot only when we hit 45 mph when we hit 55 mph it will stop. It also will stop if we turn off the overdrive. The Gears all go in smoothly and never hesitate and we have know idea what the problem could be we have brought it to 5 Mechanics and everyone tells us something different. Please Help, Thanks