Ford F150 shift vibration

I have a 1993 Ford F-150 inline 6 that has a bad vibration after it shifts into overdrive. It only happens brtween 40 and 55 but goes away when you step on the gas. I have wiggle tested all electrical conections and check all vaccum lines. I have replaced several vaccum lines that were bad. I have tried transmission shifting products and changed the filter and fluids. Engine light is not on and no codes. U-joints have been replaced and rear end has been checked. Nothing seem to be working. Anyone have any ideas.

Thanks

does it stop if you push the clutch in ,at that speed, and coast?

sorry I forget to mention the it has an automatic

its possible its your torque converter clutch. it only engages in o.d.