[66gmcguy]

I know this is probably a dumb question because it sounds like you know what you're doing, but did you check the pinion angle with the truck sitting on the ground and level, and not up on jack stands with the rear end hanging down?

If you think it's possible it's coming from the front, it could be death wobble. I would check the ball joints and tie rod ends for slop (with the front wheels up in the air.) If it's not that, then check your steering stabilizer.