It took Chrysler a few years to de-ramblerize the XJ. I would consider the 88-90 transitional, still more AMC than Chrysler, despite some evolution and some parts takeover. As far as 4.0's go, this makes the 87 the crowning achievement of AMC. Considering that the 4.0/ AW4 combination was new that year, its durability and quality make it a pretty impressive first-year model. 91 marks the beginning of the Mopar fuel injection system, but even then it took a while before they changed over the seats and the steering column, and a few more (97) before they changed the body and interior significantly.
They all have their virtues. I think the AMC bodies seem to have been a little more resistant to rot, and a little better stamped out. My 95 is a rustbucket, and all the panels seem just a little wrinkly, despite its obviously hit-free life. The 87 looked better made, and although it rotted too, some parts, such as the rocker panels, didn't go nearly as quickly. The roof rot also seems worse on the later ones. My 95 has puddles on the floor all the time from various undetectable leaks, and has always had a cold draft under the dash that I have not been able to trace.
The AMC era paint had terrible clear coat problems, which seem to have disappeared some time in the 90's. My stepson's 93 never had this, nor does the 95.
The Mopar era AX15 5-speed is certainly an improvement over the Peugeot BA10 that AMC put in. I found both the Renix and Mopar injection systems well designed and relatively trouble free, but the Mopar has an improved crank position sensor, and the injectors don't leak so readily. The REnix era cooling system, although it works all right when it works, is a little harder to keep healthy, and the plastic pressure tank tends to crack. The Mopar OBDI is pretty handy. OBD II of course is handier still. Renix systems require a little more sleuthing sometimes to diagnose.
Wiring issues have been mentioned. The 87 had many connector and harness issues, and in general the later ones are a bit improved in that department, although all of my XJ's have had some problems with wiring, bad grounds, corroded light connectors, bad connectors, and in the case of the 93, a defective splice deep in the fuel injection harness that was a real PITA to track down. All XJ's have cheap wiring of absolutely minimal gauge, but they've improved some of the connectors over the years.
The 99 feels more solid and tight than the 95. The 95 seats were a definite step down from the 87, and the Mopar steering column sucks. The wiper switch has fewer intermittent settings, the horn buttons are never where you need them, and it's almost impossible to signal in a hurry without flicking the wipers to the wash position, whereupon they wipe the dry scratchy windshield 3 times. The previous Saginaw column was better. The 97-up column is better too.
Interior storage post 97 is awful. It was never very good, but on the 99 it's pathetic. The only thing they improved was the cupholders.
AMC era 4.0 engines seem not to have had the piston slap problems that plague the later ones. My 87 was quiet even after 200K miles. The 95 has slapped since I got it at 110K, but after another 150K it seems trivial.
The early 4.0's had overboosted power steering. Later ones have a little heavier feel. Renix era brakes were marginal, and although I've never had ABS, I hear that the ABS was particularly problematic, subject to various recalls and extra warranties. By 93, the brakes had gotten pretty strong. The brakes on the 95 and 99 are pretty powerful. All XJ rear brakes are crap, and the self adjusters a joke, but that's more a general complaint about American drum brakes. My 95 has gone over 260 thousand miles with the original drums and rotors, though!
Of course the main thing wrong with Chrysler and the XJ is that they stopped making them!
As J99XJ says, they're all XJ's and they're all good. I must take exception to one detail of his report regarding the SJ. It's true Kaiser designed it and AMC continued it with a new grille, but they also disembarrassed it of what might possibly be the worst engine ever made, certainly the worst ever put in a Jeep, and incidentally the only contribution Kaiser made to the Jeep engine line, since they kept the Willys designs or borrowed from AMC and GMC for all the others. I'm referring to the infamous "Tornado" OHC 6, which like some other historically bad engines (e.g. the Chevy Vega, the Ramber Aluminum V8, the Crosley Cobra come to mind) was a brilliant and innovative design which had a host of virtues, as long as you didn't actually use it.
