Personally I believe the I6 engines are some of the best, but this really boils down to two main points..
First, if you plan on beating the hell out of your rig as opposed to wheeling "the smart way" then that obviously will greatly reduce the life expectancy of your rig.
Second, keeping your rig well maintained will make a world of difference, and not just the engine, but all the components. A buddy of mine has a 97 XJ with 223,000 mi approx and takes great care of his rig while still hitting hard trails and has no problem with his.
I have a 99 XJ with 116,000 mi approx and am still adding many mods to mine and plan to keep it seriously for at least 8 - 10 years.
Rubi's aren't a bad base to start with, but in my opinion, a solid build + solid care taking + smart wheeling = easily 300K mi or more on a jeep or toyota. :elkgrin: