Vent problem, vaccum leak?

96 XJ Country 4.0L. When I have the air ducts on the vent setting or heater setting the air louver seems to re-direct up to the defrost location based on Engine RPM or Engine Vaacum. It seems like a vaccum issue. Any ideas where to look to resolve this problem? TIA

When vacuum is lost, it defaults to defrost, so I think you're on the right track. Two usual suspects are the vacuum reservoir, located behind the front bumper on the right, and the line to it, which runs beneath the battery tray, and is often attacked by leakage.