i have a problem with my a/c when 'm driving along the air comes out of the defrost then shifts back to a/c vents .i replaced the control switch no luck . help please

 

You have a vacuum leak somewhere. If you have cruise control check the hoses under the battery tray, if it's a 4x4, check the line underneath that got to the front axle. If those things are OK, check the hoses and the check valves under the hood next to the intake manifold.