Horrible 4wd Grinding Noise

I have a horrible grinding noise in 4 Hi (no noise in 2 Hi, havent been in 4 low for a while), doesnt matter if it's on snowy streets or dirt roads (I, of course, dont use 4wd on dry pavement) But when I have it in 4 Hi, I get a grinding sound coming from the front end (definately isnt t-case, you can tell it's coming from further up front) that sounds almost exactly like I have a snow plow on my Jeep touching pavement.

I've tried twisting the u-joints on the driveshaft and get no play, and I recently changed the front diff fluid & gasket, and there was absolutely no metal shavings nor irregular wear anywhere. I also checked the drivers side axle shaft U-joint when I changed the wheel bearing a couple of days ago, and it seemed okay. Thought maybe the slip yoke on the DS was getting a little dry, so I put plenty of grease through the zerk and the noise still happens. Could the u-joints be failing without any play? Or maybe the slip yoke in the DS is bad? It sounds like it's coming from the center of the vehicle, but it is kind of hard to tell when driving down the street. It has been this way for quite a while, and steadily getting louder, but havent been able to track down the problem.

'89 Cherokee Sport, NP231, AW4, 4.0, Disco-D30 front axle.

The front axle U-joints usually get the loudest during a turn in 4X. They sometimes get dry and make noise, then fail and get play. A hub bearing gong out will make a racket, but all the time, irregardless of 4X or 2X.
Is it possible the axle isn't engauging all the way and the splines are rubbing?. An easy way to check would be to jack up the right axle (leave the left tire on the ground) and turn the tire by hand and see if the driveshaft wants to turn. You'll have to run the motor a bit for vacuum for the axle shift, in 4X, tranny in neutral. *Block the rear tires*
Grab the yoke at the rear of the diff and try moving it from side to side, use some muscle. Look and see if gear oil is seeping out of the seal.
If the carrier bearings are going out, they usually howl. I try to pry the carrier up, most any time I'm in the case, to check for bearing play.
Maybe jack both front tires of the ground, put the tranny in neutral, the transfer in 4X (you'll have to run the motor some to get vacuum for the axle shift), then turn off the motor and rotate things by hand and listen.