So, I changed the oil in both diffs the other day, and was a little surprised to see that the oil in the rear was much cleaner than the front. I chalked it up to the PO maybe not understanding that both diffs are turning all the time and only changed the oil in one. Well, today I was driving around a corner and the rear end popped, as mechanical lockers sometimes do. I've noticed a ratcheting sound from the rear since I got it, but it's nowhere near as obnoxious as others I've heard, so I dismissed it. I jacked up the rear end today and put 'er in Neutral and lo and behold, both rear wheels spin the same way. It's locked, right? The only other vehicles I've driven with mechanical lockers (Lock Rite) would get bound up on corners and pop and release quite a bit, and the ratcheting was much more noticeable, but they were very lightweight vehicles (Jeep XJ and a Samurai).

 

Oh yeah, so I need a different type if gear oil back there? I've seen some with additives for limited slip rear ends. Just 85/90 in there now.

 

If you're hearing ratcheting, then it's likely a Detroit. No special gear oil needed for it.