'98 AC Vacuum problem

I have a 1998 Ranger that has been working great for the last 7 years until this unbearably hot summer. Now when I start accelerating AND the truck is moving the A/C only pumps out the defroster and lower levels. A friend of mine says that it may be a vacuum problem and that it may be impossible to find. It doesn't happen if I just rev the engine, only when I apply the gas and try to get on the highway. Once I get to my cruising speed I slightly release on the gas pedal and the AC starts blowing out the vents. Any ideas on where to start looking?