My problem is... When I have it in 4-wheel or auto I'm getting a howling noise. (Pretty Loud) When I'm in 2-wheel I can still hear the howling but it's very very faint. I'll tell you what I've done so far.... I replaced the front u-joints and put new bearings and seals in the front differential. Any ideas?

Could be uneven tire wear or could be internal problem with one of the differentials.

When I have the truck in the air it still makes the howl. Tires are not on the ground! The sound is coming from the front of the truck.

well i was going to say tie rods are shot because if they were the tire's whould ride with lots of toe in or toe out causing tons of tire road noise but since you still have it with wheels of the ground i say check the front wheel bearing assembly and they are not servicable so they entire hub would need to be changed and sorry but these babies are not cheap! good luck

I've checked the wheel bearings and all look good there. I'm thinking ring a pinion?

If it does it in the air sounds like a worn, damaged or inproperly setup ring and pinion.

It sounds like something is worn in the diff. But when everything was apart it all looked good.

Front wheel bearings?? I thought they were sealed constant velocity joints. And unserviceable like Busta stated. I could be wrong for the year/model, but 98 1500 4WD's are sealed CV joints.