It sounds like a simple vacuum check valve has gone bad.  As I get older
and the memory deteriorates, posts like this help me remember why I don't
want to buy a new truck again or have anything to do with dealers.

Yep...  check for vacuum leaks - common areas are the cruise control
diaphragm that sits under the battery tray (acid drips on the vaccum lines
and eats them away), the vacuum line going to your your transfer case, and
the lines from the case to the front axle disconnect (if 4x4).  As BIR
suggested, it could also be the check valve on the line that runs to your
HVAC controls.  Find the vacuum line off the back of your intake manifold,
and look for the check valve.  Note which side connects to the engine, and
which goes towards the passenger compartment.  Remove the valve, and try
pulling a vacuum (use a hand vacuum pump, or, in a pinch, suck it like a
straw) from the engine side.  Air should flow freely.  Do the same on the
other side.  There should be restriction (no air should flow).  If it does,
the check valve is bad.

Get yourself a couple of rolls of vac hose and start changing them out.
You're sure to find cracked hoses in a 16 year old truck. The cruise
diaphragm is a good bet too.

BDK

I had a '94 doing the same thing. They come from the factory that way.
There is a fix that increases the size of the little vacuum canister
next to the throttle body. The factory one is about the size of your
little finger, the fix is about the size of a 35mm file canister.
Someone posted the part number and connectors for the fix on line but I
erased my copy when I installed it. Parts cost about $10 and 10 minutes
to put it on. No more problems. When you hit the throttle the little
cannister doesn't provide the vacuum buffer and the vents change from
dash to windshield.
Si