If the a/c unit is too large for the room, it won't run long enough to adequately dehumidify the room (i.e., it may be short cycling). Another thing you could do is add a fresh air recycling system. This basically has a heat exchanger (and some have a moisture exchanger) to exchange interior air with external air. If the house is tight, running the vent fan won't do much - to exhaust air, air has to be coming in from somewhere.
Is the crawl space got any plastic on the ground, or is it open dirt? If not, ground moisture will raise the room humidity.