Drive it till it breaks, don't rebuild it before that.
A properly maintained and never overheated 4.0 can be expected to go over 200k before major work.....If she then still has good compression and not burning oil, just keep going
If at 100k, you still have great compression and not burning oil, then install new top quality water pump and timing chain and gears, and spark plugs and wires, belt and other minor stuff.
Take care to prevent overheating and cooling system corrosion, and keep your oil fresh...you'll be fine