"illin'"-gate

I think the issue isn't whether "illin'" means something bad or good, since it's well-attested both ways, much like "bad" itself. The important distinction, to me, maps onto something like the "ser/estar" distinction in Spanish. In that case, "illin'" would be an "estar"-type word, and "wack" would be a "ser"-type word. If I say you're "illin'" I'm referring to your current state, but if I say you're "wack" I am referring to your essence. I don't think the two are directly substitutable and therefore, they are not synonyms.