You have to have the truck in Neutral to put it in four wheel drive obviously, and as long as "4WD LOW" or "4WD" is lit up, you're in four wheel drive...

I don't know much about the third gens but that's how my second gen is and I'm sure it's the same.

EDIT:
I just re-read that. Four wheel drive high is the same as "auto." It shouldn't engage the front tires until the back ones slip. Four low always uses front and rear and a lower gear range from the transfer case.