4WD/4LO transfer actuator seal. VERY expensive to replace - the whole actuator needs to be replaced - that being said, there is a way to do it yourself, but you must be quite experienced to do it. I just had to replace mine because my actuator for 4Lo was seized. $2300 - the only way to replace that seal is to open up the transfer case and remove the actuator - which must then be replaced. Check the forums for more information on the 4x4 actuator.
