Why would a 1998 Lincoln Navigator 4 WD jerk when making a turn in 4 WD?

NOT a GOOD Idea 2 b on a Hard Surface with 4WD Engaged while Turning----If the Gear Oil in the Different-- ial is OLD-SOMETIMES U will get a Chatter or CLUNKING-When U change the Gear Oil PUT in an ADDITIVE 4 LIMITED____ SLIP Rear & Front ENDS.(Differentials) Marv, Thanks for the advise to this question...didn't understand the last part of your answer which stated"slip rear and front ends (differentials)" What does that mean?