I had a problem with the PCV elbow that connect the PCV tube to the throttle body on the right side near the front of the intake manifold. The rubber gets soft and cracks. My idle was low and slightly rough. It would even die if I let it idle long enough.
I used starter fluid to check for the vacuum leak there and it checked positive for a vacuum leak. The propane method did not work for some reason.
This seems to be a common failure for the 4.0 SOHC engine and a few other Ford engines. It can't hurt to check it out and you may find your problem. It is kind of under the top radiator hose.
2002 FX4 Bright Red Supercab 4.0 SOHC, manual 5-spd & Tcase, 33" BFG MTs, K&N air filter, Modified air box inlet, Gibson SS cat. back exhaust, Bama flash, Hidden Hitch round receiver, Rear helper air springs, Electric trailer brake controller, Line-X bed liner, WAAG bar with KC lights and Wildernest shell.