Most probably the locking hub: do you still have the original vacuum actuated hubs? If so disconnect the vacuum supply (vacuum to hubs)
www.geocities.com/lmp4203/kiasport/vacsol.jpg and see what happens. QUite often...I'd say not a question of IF but a question of WHEN, on 2WD, the vacuum solenoid valve starts leaking and applies partial vacuum to the hub that will not engage completely and the 2 sides rub and grind against each other, and the hub is shot....At times, it will finally engage completely...thus the clunk.
Disconnecting the supply will prevent partial engagement...but the hub is still shot. Replace with WARN manuals or auto-mechanical
http://www.geocities.com/lmp4203/kia...hubinstall.pdf
In the mean time, if you really need 4WD often and you do not care about increased fuel burn, then you can disassemble the hubs , reverse the position of the spring to the other side of the piston to keep them engaged all the time, and reassemble. 2/4WD will still be available at transfer case, but the front shafts will rotate all the time, increasing noise and drag. (this would be like manual hubs kept in the engaged position) . Come back for details if needed.
less probable but possible:
Any changes in position or feel of the brake pedal? WHen brake pads start to desintegrate, they generate various kinds of signals....
...OR I could think of bearing failure, someting reported quite often.