AC works fine until 2000 rpms then it switches to defrost vents

Hello, motors controlled by engine vacuum area utilized to direct air flow to the proper areas. Since increasing engine speed changes engine vacuum, it is likely that there is either a vacuum leak somewhere under the dash, or perhaps the vacuum controlled motor is weak and needs to be replaced. Testing and physical inspection is needed.

HVAC vacuum supply leak is commonly found under hood in the small black vacuum tubing near passenger side firewall..