weird high pitched wining sound coming from front.

I noticed the other day when driving near a wall that something on my 2000 xj sport was making a high wining sound kinda like an electric motor winding sound of some sorts. it gets higher as i go faster and lower when slowing down. it does not start making the noise until ive been driving for 5 or 10 mins it seems. the wining seems to stop when im going 15 mph and less. it definately sounds like its from the front and left but im not totally sure. i'm thinkin maybe front differential or front driveshaft( which i lubed today with no result)or maybe even bearings. Does this sound familiar to anyone? I checked my differential fluid in the front it seemed to be at a good level but was black as the night and smelled real strong. My jeep is a 2000 sport auto 4x4.

Thanks,

PS. Why is there a hose that runs from the top of the diff pumpkin to under the hood with a plug in it?

My guess on the whining sound is the front drive line. Specifically the CV joint in the fron DL. Unless you have been greasing the Ujoints along with the ball every time you change oil I bet it has gone bad.
As for the oil in the diff, take the cover off and change it. Look around while you are in there and see if there is anything that does not look right. Use Permatex Blue gasket maker around the cover before putting it back on and 80W-90 weight gear oil to fill it with. Take my word, this is a very easy task and you can ask me how I know...

yeah i was thinking about changing the fluid, what would be a good way to pinpoint exactly where its coming from? i was thinking maybe getting it up on 4 jack stands after driving it long enough to get it to make the sound. then popping it into 4 and letting the wheels spin. also will the black high temp permatex gasket work just as well?

Could it be an Idler or alt bearing, Can you make the whining noise with added RPM's sitting still not moving?

I figure the 5 mins is the time it takes to get the bearing hot.

And the tube from the diff to the firewall is a breather/vent so the differential doesn't get water/mud in it.

it only makes the noise when its rolling if im in park or nuetral i rev it up and dont hear it. also ideally, what's a healthy color for diff fluid? im pretty moderate at wrenching on the jeeps. how hard is it to change cv joints on the front driveline? if i should need to do so

do these jeeps have cj joints, i thought that was only in fwd cars with independent suspension?

its a double cardon joint in the 87 -01 models.