It sounds like a classic case of carbon knock since it stops when the throttle is backed off. Hence my response to fill the tank up with fresh gas and pour in two bottles of Chevron cleaner.
In cases of extreme carbon build up the engine really needs to be MotorVac'd which isn't terribly expensive.

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2000 XLT 4WD 4.0L OHV, 5 spd, Alabama Cylinder heads, Delta Cams Valve Train and Pushrods, JBA Headers, KKM Cone Air Cleaner, BBK Throttle body, Dynomax Cat-Back exhaust, Summit Racing drilled front rotors w/ EBC pads, Cobra rear disk brake conversion w/ Hawk pads, 8.8" 4.10 Traction-Lok