Is this cavity ever dry? In real life?
TL;DR - The seal in the transmission that accepts the input shaft from the transmission is leaking and needs to be replaced.
At the same time I noticed the slow leak at the rear diff, I also noticed a drop of gear oil on the transfer case skid plate. A quick inspection led to the seam between the transfer case and transmission - a cavity that should be dry. More often than not, a bit of oil leaks into this location, with the probability increasing if the two components have ever been separated.
After noticing the gear oil, I added a bit of oil to both the transfer case and transmission and then proceeded to drive a few thousand miles, checking the oil levels again upon my return. Only the transmission was lower than I'd left it, making it the prime suspect.
Given that I replaced the transfer case about a year ago, the likely culprit is the seal on the transmission side that accepts the input shaft of the transfer case (90311-40007), so I've ordered a new one and will replace it at the same time I work on the rear diff.
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