front axle noise

when 4x4 is engaged there is a bad "clang" noise coming from the front end.  it makes the noise initially when put from drive to reverse, or vise versa, and it continues as driving.  changed transfer case fluid, checked drive shaft, u-joints SEEM to be ok. u-joints at the hub seem alright too.  only thing i can see out of place without taking apart hubs is there is a round piece of rubber on the inner side of hub on both sides of axle that has came out of position. sorry i can't explain it too well, hopefully someone knows what im trying to say.
thanks for any help