Bump - Same Issue + a New Question...
I'm having the same issue on my 94 4.0. It's intermittent, so tough to properly diagnose. Just did the valve cover gaskets but the starter is generously covered with sludge, so I'm thinking that's where the problem is (i.e. bad connections, plus possibly oil/goo in the starter-mounted solenoid). When it doesn't start, I do hear a click that sounds like it's coming from the relay inside the driver-side fender.
I had this problem maybe 4 years ago and a starter swap fixed it. But I'd rather not pay for a new starter if I can just clean up the old one and possibly swap out the starter-mounted solenoid to get it starting consistently again. Anyone know if this is possible?
-Pete