All has already been posted on this. I'd search or scan the past posts.
87-91 has the standard CJ type rollbar, 92-95 has the full roll bar for rear shoulder belts (great for the safari top to cover the back as well).
I think that 87-90 were TBI (throttle body injection) and 91-95 are fuel injected.
Also, I'd shy away from the earier years with the 87-89 or 90 because of crappy transmissions. Like somebody else posted once, you might as well go for the later years when most of the bugs have been worked out.
I'm likely missing something or wrong somewhere, but this info will give you an idea and the others will fill in the rest.
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92 Canyon Blue Wrangler YJ 4.0L - with some fun goodies