A/C will only blow out of defrost. The air will not blow out of vents.
Why does a/c blow out of defrost? on 2003 Ford F-150
Go to this web site and following the instructions. Its simple and it fixes the problem. Cost very little. I did it and it did fix the same problem your having.
You have a vacuum leak somewhere in the dash. Check all vacuum hoses under the dash to make sure they are connected.