Trans or torq ???

2000 F150 4wd 5.4
Was driving up a 12% grade was cruising at 50mph and near the top the truck all of a sudden started to shake or vibrate, after topping the hill I let it coast down the other side . It coasted just smooth as silk at the bottom when I engaged the motor the vibration started up again. So what am I looking at trans or torque converter?
All new plugs, coils, injectors no engine light no codes and passed a missfire test on scanner. 0 missfires on all 8 cylinders . not an engine problem.

Check your fluid level.

Does it do it in all gears?

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