F 350 issue not cooling while vehicle moving

My f-350 A/C works when at idle but when driving down road temp in vents keeps rising all the pressures look good at idle any suggestions! The heater valve is closed for

Try checking at 1000 RPM and see what pressures are doing. Also is the engine fan electric of cutch fan? Sometimes they go bad and need replacing. If fan goes bad when moving slow in traffic or up hills it will impact A/C operation. Likewise low refrigerant may appear in normal range at idle but not above. If freezing up definitely I slow on refrigerant or a restriction may be present somewhere. Could be a number of things but these are easy checks before getting into the expensive items.