Originally Posted by Overdriven

Maybe your clutch is slipping because the bell you pulled out of your box was contaminated or glazed over. This would cause the motor to rev up and not shift under load. Take away the load by lifting the truck in the air and the clutch isn't working as hard, so won't slip as much and will be able to drive the trans fast enough to shift. Take the bell off and use some sandpaper to rough up where the clutch shoes contact it. You can also scuff up the clutch shoes a little and use some degreaser on everything to make sure there's no oil on anything. Double check the slipper clutch too, if it's slipping to much you'll have the same problem.