[browning]

1989 grand cherokee auto..5.9 motor ..was backing up and rear diff. made a crunching sound ..checked the oil in diff. and it was black ..i drained the oil and replenished it with 80w90 synthetic oil ...noise in diff. went away so far so good ..[is this a common problem ???? ]

[GrouchyTim]

Was there anything on your magnet? Did you do any work in the previous few thousand miles that you may have left something in the pumkin? (once found a flashlight battery in one-that was interesting) No that isn't common and you may want to pull the diff cover and visual inspect the gears. Check your tooth markings and backlash and pinion teeth.

[browning]

i did not remove the diff. cover i siphoned the oil with my vacuum pump ...and replenished it with new oil ..i know i should have removed the cover and cleaned everything ...lets wait and see ...