98 xj 5 speed 4wd

Just this morning while going down the interstate, and sounded exactly like a serp belt squealing. I got to a stop light and it went away. It only does it when I'm moving, and it gets faster and more aggressive the faster the vehicle goes. I greased every u joint and bushing with a nipple on the jeep. That had no affect. It sounds more towards the front.
Any ideas? There is no vibrations and it drives the same as it always has.

 

try putting it into neutral while driving at speed. If it goes away, then its a belt (you can verify by revving up slowly to the same RPM while its moving; if it comes back, belt. If not, more work needed). Have you checked to see if something is rubbing the tires / front end? Right amount of fluid in the axles?

 

Let's start simple (greasing nipples while something sounds like a belt likely doesn't resolve the squeal).
What's your belt tension like at the moment? It needs to be pretty snug in the XJ.