The bearings are damaged.
In the differential
The front differential uses 80W90 gear oil.
YesYes
It could be differential bearings.
The bearing are sealed and can not be repacked.
Probably a breather pipe for the front differential.
It is in the center of the cover on the rear.
The lock actuator has failed on whichever doors are making the noise. The actuater is serviced with the door latch.
You can find the schematics for a front differential on a Jeep Grand Cherokee 1994 at various auto part stores. You can also check in the owner's manual.
No, use ATF only.
The company that makes the Grand Cherokee is the American company Chrysler, they produce it under the JEEP division. They have been making the Grand Cherokee since 1983.
Wheel bearings, and if they're making that kind of noise your spindles could be shot by now too.