I had the same problem with mine a while back. This is what I did:
I started with the hubs and found that both of my lines were broke so I replaced them. Then I when in to the engine bay and found the vacuum lines on the passenger side fender wall. On the bottom of the vacuum actuator there are two lines coming off of it. One runs in to a t and then in the canister. The other one runs south to a t on the frame rail and then to your hubs. I could almost bet that there is a leak between your hubs and the actuator or your actuator is bad.
I took the two lines off of the bottom and plugged the bottom one (this is the line that runs to your hubs) with a rubber glove that i had in the truck, waited about a min or so a boom I now have ac coming out of the vents not the defrost. I hope this helps here is also a picture of the vacuum actuator.