How many miles on your 4.0's?
#307
Just turned over 300,000 - but that's in kilometers - about 190,000 miles more or less - all original. No major engine or transmission problems, except that I have gone through 2 alternators.
Regular routine/preventative maintenance is the key, IMO, if you want to keep a vehicle running for a long time (that is, assuming that your vehicle wasn't a "lemon" to begin with - though these are rarer than they used to be).
Parts will wear out, problems will arise - fix them early, and you will be better off in the long run.