Toyota Repair Problems? Ask a Mechanic for Answers ASAP
The noise may or may not be related to the 4wd front differential not engaging, this truck uses "ADD" auto disconnect differential which means the front driveshaft is turning all the time, when 4x4 is engaged the front differential actuator engages the front differential to receive torque from the driveshaft to turn the front axles and 4wheel drive occurs.
In most cases the front actuator is not getting engaged because there is a vacuum line disconnected from the diaphragm or the actuator has a leaking diaphragm. The first thing I would suggest is removing the fill plug and check the oil level and add 90w gear oil as needed, then the vacuum hoses should be checked to make sure they are connected to the actuator as well as the vacuum solenoid under the hood at the right side fender, if no problem there check the vacuum pipes on the actuator to see if they may be clogged with rust, use a small steel wire to clean out the ports, if nothing else can be found remove the actuator assembly and apply vacuum to one of the ports and then the other using a vacuum pump, the actuator shift fork should move back and forth when vacuum is applied.