I would verify the crank position sensor before you start throwing parts at it. The CPK tells the PCM to tell the coil packs when to fire.
Search or ask in the WJ Grand Cherokee Tech Forum here:
Look and/or ask about checking and/or replacing the crankshaft position sensor and go from there.
As for a vacuum leak (oil leak), if it is an oil leak from a gasket such as the valve cover it could possibly allow extra air to enter through the intake. BUT, that air enters BEFORE the throttle body and will not generally cause a lean condition. The crankcase ventilation system is designed to work that way. What it does draw in is burned off during combustion. It is part of the pollution controls of a vehicle.
An intake vacuum leak, on the other hand, will affect how an engine runs but will not generally cause a misfire in newer vehicles.
Oh, and forget the "voltage pen". You have a system with coil packs and there are no plug wires to check for spark passing through. Also those testers are the most basic and will not work on auto electric systems. They need a minimum voltage level to detect (12 volts is too low).
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