I have a 2004 Dodge diesel that blows cold air on the driver side but warm air on the passenger side. I do not have dual climate control. What is the problem and how hard is it to fix?

the system is a little low...shoot a little freon to it and it'll work for another year or so....

my old '03 diesel and my buddys '04 diesel does/did the same thing...

I've tried to figgure out why it does this and for the life of me I can't, but just shoot a little juice to it....