Front axle/differential 4x4 making howling noise

There could be four different issues. out of oil, bearings are in need of replacement(could be also of the previous), possibly even out of time or teeth are needing adjusted for proper meshing.
I am not there to hear the howl so i cannot determine perfectly for you.
you will need to any any of these cases remove the differential cover and inspect interior of housing and oil if any remaining for metal and worn parts.
I would suggest getting the repair manual for your vehicle as this can be a difficult process and can cause serious issues if not done correctly.
If i am remembering correctly these are not a hunt/ search differential. meaning they don't have a specific setting point for gearing.
If they are set timing they will have indents on gears that must be aligned when assembled.
also check for excessive movement of pinion gear.(signs of wear and needed maintenance such as excessive oil leakage.)
also when the book states a certain amount of foot or inch pounds to tighten the pinion it does not mean to tighten it that much. it means when you spin the pinion gear that is how much force required to keep turning it.(as you are turning the pinion the gauge should read that amount)
in other words if you are turning the pinion you should be applying ## foot or inch lbs. so you would tighten the retaining nut until this point is achieved.

Is it making the noise constant driving down road or does it noise move from side to side when you turn the wheel going down the road? I would consider the bearings going.