I'm not familiar with the 2.8, but first thing might be to check if there is a ballast resistor on the coil. If there is, try temporarily bypassing it and replace it if this solves the problem.
Note that some 4.0's have a ballast resistor in the fuel pump circuit simply to quiet the pump, and can be run without it, but if you have one on the coil, its purpose is to provide a fatter spark for starting, and if you run without it it may burn up the coil, so replace it.
Another possibility here is a defective ignition switch. On XJ's this is on the steering column, down near the base of the dash panel, operated by a rod from the lock. It's possible that if the switch is worn, it will provide voltage to the ignition when in the start position, but lose it in the run position. You can check this with a test light on the positive lead to the coil or if you can find the pertinent terminal on the ignition switch itself, test it there.