At this point, I think the issue is some kind of transmission hunting, especially considering it seems to happen right before where the transmission would normally shift. The transmission seems like it's "confused" during part-throttle, low-RPM cruising near it's shift point, almost like it can't decide whether to upshift or hold the current gear.
If anyone has any imput here, I'm all ears. The issue was lessened with a new TPS. The throttle body adjustment didn't have any effect on the problem, although the increased cable tension makes the truck much snappier off the line, which I like.