Have u checked your slipper clutch to see if it is set too loose ??
Are the slipper pads good ?
How old is the truck ??
Are your Clutch shoes worn ?
And something i would ask myself would be, is the "primary clutch" (second gear clutch inside the transmission) good or is it worn ? is it oily or slipping.
What you describe sounds like something in the system is either slipping when in second gear or that something is worn giving the sypmtoms of slipping when in second gear and at high RPM.
I had the same thing until i changed out the primary clutch, slipper ring and pads and the clutch shoes on the flywheel. That is, until i blew my tranny and had to swap it for my spare.