On the 99 Silverado new style there are No locking hubs as the axles are splined and locked by a large nut. The axles will always turn as you drive and the ring and pinon will stay disconnected until you engage 4hi or 4lo. When engaging 4hi the front axles and carrier are connected by the actuator pushing and holding a rod in. The transfer case shifts to the selected gear and connects the power from the transmission to both the front and rear drives. The front is driven by clutches in the transfer case that do wear out when abused by not changing the oil or putting in the incorrect oil. Electronics take Auto-Trak II blue oil, hand shift take regular tansmission oil (pink). So all in all said check the fluid first and replace if black, but that may not correct problem as the clutches may already be damaged by excessive driving habits, driving 4x4 on dry pavement etc.

 

I still have yet to figure this out.. i dont think the transfer case is engaging, maybe a vacuum? or linkage?