Don't know if it applies to other years/engines, but I seem to recall reading about a Ford TSB about a similar vibration/noise problem from the gas pedal on my year Ranger - it was linked primarily to the cruise control set at a certain speed.
Seems to me that it must be related to the throttle cable - you may want to see if there's excessive play in the cable at it routes to the throttle body - e.g. maybe tie it down or cushion it anywhere where it seems it may have extra movement.
Hope this can help.
'03 Ranger Supercab, Flareside, 4.0 liter V6 SOHC, auto, 4 x 4 w/FX4 Off-Road Package (Level I)