91-99 are all good reliability wise.
All 91+ 4.0L have the H.O. engine, and they can have the Chrysler 8.25" rear though they can also have the Dana 35 (weaker), 96.5+ with the 8.25" have 29 spline shafts vs. 27 spline shafts on older models (29 spline are slightly stronger).
97+ have the updated body (new grill, etc.), and a nicer interior.
00-01 suffer from the 0331 head issue (known to crack if overheated), this applies to all 4.0L I6s not just XJs. 00-01s also have a Low-Pinion Dana 30 in the front vs. the High Pinion Dana 30 in earlier models. The LP D30 can be an issue if you plan on doing a lift especially if it is over 3".
96 and 97 were transition years so there are some "weird" things with these years (not necessarily bad, but have year specific things so you have to be more careful when buying parts).
98-99 are often considered the "best" years but they aren't significantly "better" reliability wise than other years in the 91-99 range, but they are the newest models without the 0331 head and have the nicer interior, later "features", etc.
With 91-99s the biggest thing to look for is rust, especially in the floors and underneath. With 00-01s it is important to also look for signs of a cracked head ("milkshake" oil, oil in coolant, coolant in oil, stop leak in the radiator or recovery tank, etc.).