I have a '91 YJ, too, with the 4.0 liter engine, and it's been a reliable vehicle for me for these last 20 years. I had to have the A/C compressor replaced last year and had to have the head and valves reworked earlier this year, but it had almost 200,000 miles on it at the time.
The good thing is I could take it to a local mechanic I trust (rather than the dealer whose service department I don't trust at all) because the engine is dead simple to work on and there's plenty of room under the hood to actually get to whatever needs to be worked on.
It's been in 3 accidents (the first one caused about $5,000 worth of damage back in '92; the others were less severe) and I walked away unhurt all three times. Since it's constructed body-on-frame they were just able to take off the damaged body panels and replace them.
Oh, and just for peace of mind I'd recommend getting a hood lock. Some thieves rolled up early in the morning at my parents' house when I was staying with them, unbolted the hood, and took off with it. It's had a lock on it ever since.
2012 JK Sport S
Deep Cherry Red Crystal Pearl
6 spd manual, hard top, limited-slip diff, 3.73, Power Convenience Group
(Also have a black '91 YJ with 201,000+ miles that I bought new)