Guys whose only advice to you is "buy a 99" aren't really helping you much. Some of them just blindly regurgitate that because they read it somewhere when researching Jeeps, and they don't even know why. There are some reasons that 99 is a particularly favored year, but it might be that NONE of them matter to YOU. I mean, what do you want to do with your Cherokee? Is it just going to be a daily driver? Will you wheel it? If so, how often? How hard? What kind (mud, sand, rocks)? Are there any particular modifications/upgrades you'd want to do to it?

I think some people's obsession with 99 is a bit disrespectful to how great all the other XJ's are. Even the 00 and 01 XJ's are worthy of respect that is often denied to them by many. You really can't lose. Many minor and some major changes were made between 84 and 01. I'm sure you can find a list of all the changes out there somewhere; I know I found that info myself once upon a time.

Aside from the normal stuff to look out for with ANY used car, be aware that 84-86 XJ's had GM engines that pale in comparison to the 4.0 of 87-01 (I'm just ignoring the 4 cyl engines that some XJ's had). The pre-91 Renix systems are a bit ... "special" and simply something to be aware of and ready to work with if you're looking at one. 00-01 4.0's have a head with a casting flaw that may or may not crack between cyl 3 and 4. Many of them never crack (or at least haven't after hundreds of thousands of miles), but many do. It's something to watch-out for but not something that should keep you away completely, and you should be aware that identical heads without the casting flaw are out there should you have a crack. The manual trannies are solid as are the autos. The two possible transfer cases are both great. There are a variety of rear ends each with their own advantages and disadvantages, and unless you're planning on doing lots of wheeling, even the weakest of the bunch (D35) will be just peachy. The floor pans are easily the most susceptible to rust on all years. Because of the limited space for a radiator, XJ's are just barely able to maintain their operating temperature, so you have very little room to slack on maintaining the cooling system; luckily the cooling system is a reliable as any other part of the 4.0, so it's not likely to require much work (and any work will be relatively cheap). The stock exhaust manifold cracks easily which can impact the performance and efficiency of your engine negatively. APN makes a replacement header that is a very affordable replacement should you ever need/want one (there are other header options out there as well).

It is my opinion that if you get an XJ with a 4.0 from 91-99, you have virtually nothing but cheap regular maintenance to worry about. If you get an 00 or 01, you just have to make sure to never overheat it (as if you'd ever be OK with overheating any engine), and to pay closer attention to the oil and coolant "health."