AC problem

I have a 98 venture with AC problems, If I use the AC in position 1 or 2 the air comes cold, but if I use 3,4 or 5 it blows warm air like if the A/c does not work at all. And this happens with the air coming from 2 of the 4 vents, the other 2 work file all the times and in all the positions, this is strange.... I don't have rear AC and the system is fully charged. Thanks.