A/C blower switches to floor vents when on MAX and Accelerating. on 1994 Dodge Ram Van B250

I'm pretty sure its a vaccuum problem but have no idea where to look. I would be happy to make it stay on the dash vents only. When on normal it is fine, just on Max does it switch upon accelerating.. It's really hot here so any help would be greatly apperciated.

Asked by for the 1994 Dodge Ram Van B250