I have a 2002 4.0L 4WD auto, FX4, all factory. Does yours have the same selector knob on the dash for 2WD, 4WD Hi, 4WD Lo?
I've not had a problem with mine but agree with the advise to periodically activate the 4WD system and run around in 4WD hi and 4WD lo for a while to help keep everything...