Vibration when shifting

Hey guys, I've noticed my truck, more the steering wheel vibrates when the truck shifts from second to third. all other gears are fine. you feel the vibration when you're accelerating hard. i don't notice it when driving slow. I've noticed this for quite something. its not getting worse. when shifting from second to third manually, it doesn't do it. two times I took it to the dealer and I got the usual "could not duplicate". I have 50K miles on her. changed the tranny fluid once, not a full flush, just empty the pan and refill. Any ideas? someone said it might be the slack in the drivetrain??