your 4x4 locking hubs
in 98-00 rangers they are activated by a vacuum pulse--yeah i know, how clever ford, but yeah. they are known to fail due to leaks, cuz of bad sealings.
in the tech library u will find a how to, to lock em permanently, like ford did on the 01+ Rangers. if u have basic tools and basic mechanic skills u can get it done. that way ur gonna be 100% sure they wont fail when u need 4X4