Mike,
On the 88 XJ the electric fan is turned on when the AC clutch is engaged OR when the temp in the cold tank of the radiator reaches 190F. You will see the swich located between the two ATF cooler lines, it has approx a 1-1/4 hex on it.
There is a relay that controls the fan operation. I would start there. If you plan on keeping this XJ then call the 800 number for Chrysler techauthority and buy the electrical troubleshooting manual, it cost $20. Less than a year ago I checked and chrysler still had all the 88 manuals available.
I would trace the circuit with a multimeter and verify the operation.
You may want ot unplug the cold tank sensor for the electric fan and jumper the pins with the engine off then start the engine to see if your fan comes on. If it does not come on then I'd take a look at the relay. If it does come on that tells you it in the portion the AC system controls.
Get the AMC/Jeep troubleshooting manual, it's worth it's weight in gold. I'll never give mine up as long as I hae my 88 XJ.
Martin