Wierd vent problem

For a while now the air (hot or cold) blowing out the dash vents would suddenly blow only through the defroster vent when the engine was under load. Never been a problem since it returned to normal function under normal driving comditions. Today the jeep decided all air would go through the defroster vent, hot or cold, no matter where the air selector is positioned. Is this a vacuum problem? It's a '91 in decent condition. Thanks guys.

yes check the vacuum hose under the battery and at the vacuum reserve ball behind the front bumper on the passenger side

x2, and this would be a good time to relocate that vacuum reservoir from behind the bumper into the engine compartment near the passenger side firewall--eliminates a lot of vacuum line and makes things a bit less troublesome.