Mine is the black 90 XJ Rob has the silver 2001 XJ
Prior to the XJ I did my best in a 2wd VW Vanagon, slightly lifted do to slightly larger street tires (about 10% larger dia. and gas assist shocks) Ran the old van thru Death Valley and got it to places that the 4wd drive folks were amazed at, with no damage!
Got the XJ near 2 years ago after Rob got the silver XJ.
I think what is being mistaken for a compressor cycling on and off is that you hear my brakes squeaking as I let on and slightly off them. Some of the down slopes were so steep as to require both feet on the brake pedal. It took full pedal effort to hold the jeep on them hills, and Both of us have four wheel discs!