It turns out that eating rancid walnuts does, in fact, cause significant gastrointestinal distress. In the dry (most of the year) climate of California's Central Valley, I never imagined a rancid nut of any kind. Here in the hot, humid climate of Thailand, there is definitely a short shelf life for walnuts. From now on--after I throw away my remaining stock--I will be keeping my walnuts in the refrigerator.
The bright side of this affair, months of eating rancid walnuts, is that I now clearly recognize the smell of rancid walnuts. Safety first.