Speed reaches 50mph starts to shake or jerk

I have my tacoma with 96000 mile in it. Got it maintenance at a local toyota service center. They replaced spark plugs. After a few weeks I noticed that between 40 to 60 miles per hour my truck had a jerking or hesitation move. I physical check the 6 plugs...maybe it wasn't gapped correctly but they were. Also checked ingintion coils to see if they were burned or had any oil around...also looked fine except one of them was marked with an x in yellow. Scanned it as well an everything checks out or no engine light has turned on. What else could it be??