2001 Dodge Ram

I have 01 Dodge ram 4x4 quad cab with a 360ci engine. It just turned 66,000 miles. The problem is anytime I put under a load my climate control vents close. I recently installed an exhaust and cold air intake- no other engine mods. I started doing some vacuum tests, everything holds in climate control, but when I begin to test the hose down to the transfer case it fails. I have no problem shifting into 4 hi but it won't go into 4 lo easily. Does anyone have vacuum diagrams for this or any advise on where to start.
Thanks, Floyd
April 3, 2007.

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