96 B4000 4x4 question

my 4x4 seems to have stopped working. when i switch to 4x4 the drive train is powered to the front of the vehicle, but my hubs fail to lock. the one thing that i did notice is that when i pull the hub covers off, the substance that i'm assuming was lube looks pretty gross. is it possible that my hubs are so gunked up that they won't lock anymore? and if so, if i take them apart to clean them up what should i use to lube them? or do i maybe have bigger problems....? any assistance is appreciated.