You've likely got a leaky vacuum hose connection. I'd be surprised if the engine doesn't idle a little rough, as well. On accelleration, total vacuum available drops until the engine "catches up". Lots of things are triggered by instant vacuum pressure. Among the things operated by vacuum pressure are the dampers that direct air from one set of discharge registers to others - as in, from vent to defrost. About the only thing that will cause that is a leak in a vacuum line somewhere. Since it only occurs when you accellerate ( I presume it corrects itself when you level out)it is going to be tough to find. I'd start by looking at & "wiggling" every vacuume hose connection you can find. If one comes off its barb real easy, tuighten that one & check the operation. Look under the hood - standing in front of the truck, go upper left hand corner, there are some hoses there attached to the fire wall, be careful if you try to move them around as they are likely very brittle from years of heat. I easily snapped one off in my hand trying to jiggle it, then had to fix it. Also in the same area there are some other vacuum hoses hanging around down in there - I found a black vaccum hose broken down in there attached to a small vaccum check valve of some sort. I am still getting air out the vents so I dont think I have found the last leak, but was able to find some in the area descibed. Some say to remove the battery and look under there - have not tried that yet. There is a vacuum resovoir located behind your battery , it is for this condition.
When you accel your engine doesn't produce vacuum due to the throttle plate being open so a resovoir is there to hold a vacuum reserve so that when there is low vacuum produced by the engine there will be reserve to hold the vacuum motor applied preventing the air from changing direction in your dash.
When there is no vacuum the direction is at the defrost vents if you have a broken hose or no vacuum your ac would blow out the defrost all the time
But since it happens when you accel it sounds like your vacuum resovoir is cracked thus not allowing it to hold a vacuum
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