One other area to watch on 4x4s is the 'ADD' system - electric solenoids coupled with vacuum actuator on the front diff. ADD has been known to crap out (all it takes is a pinhole in a vacuum line.) Easy to disconnect the ADD and replace front hubs with manual versions by Warn (new) or Aisin (used/rebuilt.)

Jeff
96 T100 SR5 4WD Ext Cab, Warn Hubs, HD torsion bars, Bilsteins, Camper Shell, TJM bar, factory 31 x 10.5's, Powertrax No-slip rear, Recaro driver's bucket