2000 4.0 sport 4WD, front squeak when cold

I have a pulsating squeak coming from the front end that's really pronounced whenever temps drop into the 20s. After driving about five miles, it goes away. I don't get it at all if outside temps are 40 or above.

It occurs in neutral or drive, when brakes are applied and when they aren't, and appears to change in frequency with wheel speed not engine speed. There is no accompanying vibration that I can feel.

It's hard to tell, but my best guess is it is coming from the center of the vehicle.

I think some fluids needs changed or something needs lubricated. Any suggestions on where to start?

Thanks!

fan belt or a pulley?

Since you mention wheel speed, worn u-joints is the most likely source.

May as well service the front diff if it hasnt been done