I have seen this before and it was a slipping clutch, or clutch disc installed backwards
This was my immediate thought on this also. It's almost the only way a truck can suddenly rev high and not go very fast. Pull up to a telephone pole and put your bumper against it with a blanket in between. Be sure to be at a bumper brace. Put it in second and slowly let clutch out. NO GAS. Should buck a little and motor should die right away.
If you don't want to do that just think about it, does the motor and speed match when starting out in first? You should be able to sense that motor reaches high RPM way faster than truck is moving from dead stop. That is a worn out clutch.
