I think you need to double check the Panhard Bar.
While it's not 100 related, I have seen bad bars, usually bad bushing in the bar, allow enough axle movement to cause a truck to crab and make it drift. It never showed up in tire wear, except for a little scrubbing since the tires were new.
When it's sitting on the alignment rack, it never moves since it's stationary. I know it doesn't make sense on why they can't align the front end, but maybe it's a combination of both contributing to the drift.