The older unibodies (pre '93) suffer from stress cracking the most. Seems that the first time they're flexed after a lift it starts, and gets worse with time and frequency of being taken off road.
My very first XJ, an '84 had a 4.5" lift and was kinda rusty. I bought it already lifted and trail abused, then I abused it for a year. The doors and hatch wouldn't open when any of the 4 tires were flexed up, literally twisting the unibody just enough to bind everything.
My current '93 has minor cracking from the hard core 'wheeling it's seen too, but not nearly as bad. It has some pretty good distortion of the rear quarters at the corners below the rear side window and just above the rear fender lip.
Your '89 will likely show stress cracks in these areas: The black vent in the driver/passenger door sills, "A" pillar where it connects to the roof just under the drip rail, "B" pillar door post where it connects to the roof, and "C" pillar at the corners where it connects to the roof from the end of the drip rail diagonal down into the pinch seam covered by the hatch seal rubber.
If you've flexed it alot, you may see some cracks in the cowl under the front fenders below the "A" pilars and above the upper door hinges. My '93 is badly cracked there and I attribute that to having a cage and having done some pretty severe nose-up winch pulls. The cage is keeping 2/3rds of the unibody stiff, but from the cowl forward, the unibody wants to flex since the rest wont.
Another thing to try is find a ditch somewhere and slowly drop into it while keeping your arm/hand out the driver's window. Rest your elbow on the rubber window trim and put the end of your fingers along the top edge of the door frame so that the tips are touching both the top of the door and the bottom of the drip rail...... you'll feel the door move - the gap getting tighter or wider as it flexes.
Your XJ, with age and off road useage becomes more limber
.