heat only works on defrost setting

it's like there is something wrong in the air box.

With the knob set to floor, vent, or combo, on heat:
you hear the fan at the correct speed, but only feel a faint air flow at the outlets

With the knob set to defrost or floor/defrost combo, on heat:
operates as expected

With the knob set to floor, vent, or combo on cool:
operates as expected

With the knob set to defrost or floor/defrost combo, on cool:
operates as expected

This is very annoying, as the van is just off warranty (03 w 38k).
Any idea what is going on here?

I am having this exact problem I will keep an eye on this post.