Not sure the cause or what would cause my AC to flip to defrost at 55 mph give or take. It only happens at higher rate of speeds and doesn't go back to AC until I slow down. What would cause and where should I start trouble shooting first? Vacuum lines? Still weird to me that it happens at higher rate of speeds.

Thanks,
Tony

The higher the RPMs, the less vacuum your engine makes.

You are correct in assuming you have a vacuum leak somewhere. It could be in any hose or fitting. This is going to be a process of elimination.

What year XJ and engine do you have?

Yup! Vacuum leak. Start at the vacuum ball and work your way back to the firewall.