If you want it to run right then use a genuine mopar fluid, If you aren't that bothered then halfords will stock a standard diff oil, Depends on how much you care for your car?
Mt jeep wrangler has a dana 35 differential in it
Fluid level too high, water in fluid, axle overheating.
DOT 3
80w90
You should use atf +4 in your wrangler transmission
A 1991 Jeep Wrangler uses DOT 3 brake fluid.
In the differential
80w90
It was an option available from the factory.
I prefer Pennzoil synchromesh
After the Jeep is warmed up, if the creaking noise is noticable during turns at low speeds, then its probably the rear differential. Go to the dealer and have it serviced. All they do is replace the fluid and seal but they use a special lubricant additive.
Dot 3.