When I get to about 50-55mph my AC quits blowing out of the front vents and only blows out of the floor vents and front window vent. When I slow back down it blows correctly again
What would cause this?

Sounds like a vacuum leak. That little actuator that governs where the air goes is run by vacuum. The harder you step on the gas pedal, the less vacuum there is to “power” it. Take your foot off the gas, vacuum returns.

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2002 XLT 2WD EXT. Cab 3.0 222,000+ Miles

HIDs top and bottom. LEDs everywhere ( yes, even the glove box ) Fairly stock otherwise, except for a few minor things the factory forgot.

How would I fix this issue?

There are tons of places it could be leaking from, both under the hood and under the dash. Start outside under the hood with the truck running and listen. I use a rubber hose which I stick to my ear and follow anything vacuum with it. If it is that bad it should be easy to find.

There is a vacuum accumulator that the locals here call a "black globe" that can spring leaks also.

Check the vacuum assist behind the brake master cylinder and the line going to it.

Pull the glove box and look behind there for leaks.

There are other things that can leak but those are the most common.