go to the autoparts store and get a shop book over that year bronco and look up the 4wheel drive. if you have the push button with the automatic hubs the first thing i would do is get rid of the auto hubes they are junk and made with plastic gears. then i would clean the actuator and oil it up it is located on the back of the transfercase
Check fuses # 12, #17 and #18. Read the diagnostic codes (if applicable) from the shift control module, you'll need to have a shop perform this. The codes will help to determine where the problem lies.