2000 GMC Jimmy heater will not blow on feet only on defrost...?

The heater control unit operates on vacuum. You should check the vacuum lines to the engine intake manifold are attached and in good condition. Then remove the control from the dashboard, and check the condition of the actuators with a vacuum pump, you should be able to hear them open and close as you apply vacuum to the various lines. Also, check that the vacuum switch is not leaking. One of these things will be your problem.....