Ram truck A/C stuck on defrost...

I have a 98 V6 autotrans 1500 ram truck. the a/c has been stuck on defrost for some time. I have checked the vacuum hoses under the hood and they seem fine on visible inspection. I have also checked all the fuses.
What do yall think is the prob and what is the solutions.

Big thanks to all......

One time a fellow re-checked looking for a cracked line, including the power brakes and found a cracked one.

Use a vacuum gage when checking lines- sometimes the check valves go bad..