My 94 has the original and over 200K, admittedly it needs a new gear or two since its starting to humm. That's a tune meant to warn me. My guess is either you ran out of fluid due to a leaky gasket, or the previous owner never changed the stuff. You will need to check all the wheel bearings on the rear axle, and check the drive axles themselves for damage. The Differential is in need of replacement. If you intend to keep this Jeep, get a Trac-Lok (aka Limited Slip) unit and replace the one that's morphed into scrap metal. That way the wheel that is not spinning in the mud gets the power, and you get to move forward.