We just cleaned out the dryer vent with a leaf blower like this:
Unfortunately it still takes over 2 runs to dry a load. Before we did this, however, we disconnected the tube from the wall and let the tube exhaust into the laundry room (it gets very hot and humid!), and the load was perfectly dry after about 45 min. So, it seems the problem is indeed in the tubing in the walls.
Any thoughts on what to do next?