Vents only blowing out front

No matter what speed i have the heater/a-c on, the air only blows out of the front vents. The back vents work as they should. You can hear it blowing, and when you switch the setting to floorboard or defrost air still blows out the front vents just not as much if it was set to that setting. Mechanic friend didn't look at it and said from the sounds of it it was a vaccumm seal or something rather.

just started after my husband was playing with the controls - switching it from one setting to another really fast. I'm sure that is the reasoning behind it.

2002 GMC Yukon SLT