Going by what you are saying and my experience with a '73 Blazer my dad put together and I tore up as a youth, you are correct in the hi lock - hi - n - low - low lock positions though the hi lock and low lock weren't really positions as I remember. It was just a bit more movement in the shifter. Our truck had a th400 swapped in after the original th350 lost rev and was rebuilt. My dad made a stop at an auto parts place with the new rebuild in the back of a pickup and when he came out the trans was gone. So a used th400 was picked up and redone with a custom output shaft that mated up with the th400.
Are you sure the t-case in your truck is a an np203?
If you are sure has the 203 been retrofitted with a part time kit that changed the way it worked?