I would check fuel pressure. I take the guts out of
schrader valve on the fuel rail (careful it might squirt
fuel) and slide a hose over the fitting with a hose clamp.
Should be High 30's to low 40lbs. A friend had trouble
with his v6 4.0 and it turned out to be the fuel regulator.
He went to the dealer and they said they have been having
problems with 4.0 fuel regulators. I also have taken my
car to the dealer and they run that cleaner stuff through
the injectors and it does help.