NorCal1971... just disconnect both ends of your ARB air supply line (compressor end and where the line enters your diff) and blow the line out to clear any oil out of it.
Most of the 1st page comments, especially the ones about the seals being the potential leakage spot were ONLY if the vent on the rear housing was clear and operating correctly.
Like previously mentioned, the 9" differential is more problemmatic with vents since the 9" the oil sometimes has a tendency to work it's way right out the vent line and keep siphoning it out..sometimes up to a quart... there's a fix for this but that isn't your problem right now (luckily). If you need to pull the 3rd member to do any repairs it might be something to look into.
Check the VENT first, clean out the air line between the diff and the compressor with compressed air, THEN activate your ARB and determine if you can hear air leaking, if the compressor cycles off and on within a minute or so, and if you eventually get more oil traveling back up the air line to your compressor... THEN it's probably the large o-ring seals that were discussed on the first page...but call ARB before ordering o-rings or ?
Good luck