Squeaking/chirping noise first few minutes driving, its NOT the centering ball

So for quite a while now my xj has made a squeaking or chirping noise for the first few minutes its driven, only when completely cold--having sat for at least several hours. The strangest part is that the noise shows up with the gas applied, and then will go away temporarily if I take my foot off the gas, after a few seconds it will start up again and continue until I apply the gas, at which point it will temporarily go away again. It does not happen with the rear drive shaft out.

The way the symptoms manifest leads me to think that the noise is somewhere in the driveline after the transmission (AW4), the noise does not seem to be associated with RPM. I recently replaced the centering ball and seal on my rear drive shaft (actually a front shaft) which seems to have had no effect.

Any ideas on what else it could be? I'm sort of thinking that maybe the output bearing on my tcase (NP242) could be going bad? Maybe u joints? Rear axle is an 8.8 out of an Explorer fwiw.

Is it your belt.

Pretty sure its not the belt for a variety of reasons, foremost because I have to be moving for the noise to occur.

Take a close look at all the U-joints, including the two
front axle joints. If you see red dust around one, that's
probably the source of the squeaking. A bad one will
look dry and dusty...pay special attention to the front
axles.