My 2002 Jeep Wrangler heater and air conditioner work fine except that I can't get it out of defroster mode.  I've replaced the instrument cluster and it still blows up the defroster port.  Anybody got any ideas?  

The air deflector in the duct work relies on vacuum to control it. The vacuum actuators are very durable, so I would think they are fine. But the vacuum lines are less durable.

You can test the actuators by applying vacuum directly to them. If you remove the glove box, there is a coupler for all the vacuum lines (red, yellow, blue etc). Un plug it and supply vacuum with a test vacuum line hooked up directly to the intake manifold to each port. Then, if that works you have to back track the lines to either find the leak in the tubes, the vaccum storage can (under the battery) or the line from the intake to the storage can. Also there is a check valve on the manifold. Remove that and hook the vaccum supply line directly to the manifold to make sure it's not the problem.