They are not "bulletproof", in fact, they had tons of problems when they got old. Rust is the big one, both underbody and sheetmetal. If the subframe gets rusted through it's basically unsafe and should be scrapped, since it's a unibody, the frame and body are one unit. They also had electrical problems and rear leaf spring issues.
With that said, if the Jeep has a 4.0L straight six engine, they are nearly bulletproof if they were maintained. They will get a lot of sludge buildup if the oil is not changed often, but they have been known to run 250,000-400,000 miles. The problem is the auto trans usually needs rebuilt 3-4 times in that amount of time and the body and underbody rots out before that.
I love them though. I acutally just picked up a 2004 Wrangler, same issues, but it's a full frame vehicle and can be repaired. I would vote for a Wrangler if you want a Jeep.
The Grand Cherokee is also full frame and is the best vehicle Jeep makes.