The "Jeep" 4.0 wasn't a Jeep. GM was designing a new six cylinder to replace the old stovebolt in the late 50's and the first design was determined to be too heavy for what they wanted so they sold the design to AMC. Rambler built 199's, 232's, and 258's using the design and it morphed into the 4.0. The first 3 engines ate cams, typical of Chevy, I assume/hope that 4.0's got a roller cam but don't know. If that engine is the best part of a Jeep, it's not saying much.
Let me just begin this by saying I do love my Ford Ranger. However, I also own a 92 Cherokee 2 door, which I am fond of as well.
Frankly, I would say that the 4.0 is a pretty solid engine. If I recall correctly, the AMC 6 cylinder was developed off of a Nash engine, not from Chevy. (Granted, there were plenty of Chevy parts that ended up on them over the years, until the Chrysler takeover). Many of them last for hundreds of thousands of miles with little trouble besides oil changes, etc. (other than the leaky cork valve cover gaskets). Dirtman is right, once they had the stupid cylinder head change in 2000-2001, they became more prone to cracking the head even with the slightest bit of overheating. As long as you kept the coolant system clean, they would be ok.
Yes I know the frames on YJ's and TJ's rotted out almost as bad as the Toyota Tacoma's. But at least you can buy replacement frames for them.
I think the reputation for Jeep's being driven by people who are not as nice as Ford owners, has been over the past 10-15 years. I have run into my fair share of mall crawlers with 07+up Wranglers who never go off-road but still like to drive on the highway with their doors off. Some of the older Jeep owners are usually nicer.
I remember one winter at school some guy commenting about my Ranger not being lifted like his Jeep, how he could go through anything with his lifted truck. I had a chuckle watching him get stuck in the snow / ice with one front and one rear tire spinning.
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