That's correct, GM designed it to always run the fan on low. There is no "off" position. Also, if you have an A/C truck, and put the A/C on the max position, it should turn the blower on high. It also closes off two of the vacuum-operated doors so that you get recirculated air.
Another item to check is the fuse for high speed, and the blower relay. The blower relay is mounted under the hood on the A/C plenum (passenger side), and on high speed this relay puts the full 12 volts to the fan, and bypasses the resistors used for lower speeds. I'm not sure if a non-A/C truck has the same relay.
Running across the top edge of the firewall there is a power wire (orange, I think) for the blower relay, with an inline 30 amp fuse in a bullet-shaped fuse holder. For some reason this fuse holder often runs hot, and you may find that the fuse holder is melted, which will usually knock out high speed on the blower motor. You can replace it with a newer blade fuse holder and 30 amp fuse.
Bruce