I have a 99 JGC 4.0 with the same stall/restart issues and no trouble code from an OBD code reader. Try this:
Turn the ignition key on, off, on, off, then on and watch the instrument cluster where the mileage shows. If you get a code of any kind there is your problem. I had a code 0320 which is the crankshaft position sensor. They do wear out and fail, especially when the weather gets hotter. Once the system completes the internal check you will see a 'done'. And even though the OBD code reader will not display the code it can still be cleared.