2000 GMC Jimmy Heat Only Through Defrost

I have a 2000 gmc jimmy that is only blowing heat through the defrost or defrost/foot setting. When i move it to head, foot, or head/foot it only blows ice cold air. I have checked all the fuses that are associated with hvac or a/c and they all look good. and help would be appreciated.

Check your vacuum lines under hood.

Yup, one is cracked or pulled loose.

Thanks alot for the help guys i really appreciate it!