The ZJ's came with Dana 30 front differential, The rear came with either the Dana 35 or 44, The 44 model was different than others, in that it had an aluminum center section.
It sounds like you both need new bearings installed in the differentials. Most likely it’s the carrier bearings. They seam to have been a problem. I have found 3 ZJ that I have worked on with bad bearings. One had the front diff and two the rear carrier bearings go out. One was my personal ZJ.
A stethoscope can be used to pin point which bearing is making the noise. and verify if it is the differential. Also check the gear oil for metal particles. If metal is found in the gear oil, make sure all bearings are replaced. The whole differential must be cleaned completely. I have spent 2 hours cleaning the axle tubes due to metal found. They are cleaned like a rifle bore. Its lots of work, but worth it. If they are not cleaned the metal will work its way into the new bearings. Causing more problems.
One problem area I have found is the carrier bearing preload from the factory was set too tight. The preload should be about .012”, ( it was at least .020 or more from factory).
[FONT=Trebuchet MS]01' Jeep TJ (Jeep #5)
3.5 RE Custom SuperFlex, Raingler, Kilby, Currie, Banks, Borla, American Racing, Custom mods, Always more in the works.