Chevrolet Repair: 2001 chevy astro awd cargo van, chevy astro awd, chevy astro

Question
just purchased this van has 134,00 miles on it perfect looking van i knew there might be a problem didn't catch it till i got it home. well when driving below 40 miles per hour there is this sluggish feel to it intill you get past 40 miles a hour then it runs smooth as new. i pretty sure it has to do with the back axle which i noticed  it's leeking a little fluid out of the back axle. it has almost like a jerkng feel to it under 40mph highway driving you dont feel it. before i go to shopp can you give me a clue what it might be. also i can't get air flow thru the front vents for a/c and heat just thru the defrost vents no matter what i set the knob at  

Answer
As to the first part of your question, I don't see any relation to the rear axle to your problem.


As to the second question the most likely cause is a broken rubber or plastic vacuum line that goes from the engine compartment thru the firewall and in under the dash to the controller is most likely broken or disconnected.