i'm looking in the manual for this series truck and all it shows is a powershift transmission.
can you verify this?
verify the serial number also? does it begin with C024?

if indeed this is a powershift you will have to do some pressure checking to verify what is going on in the control valve and clutch packs etc. If manual shift check the shifting linkage and make sure it is not worn or damaged as mrfixit said.
If by chance it has monotrol then there are other points to check.
If could be that it may be stuck between high/low or something like that.

first need to verify what type of transmission it is, and if it is truly manual or monotrol controlled.