Is there a difference between mountain and road stems?
The biggest difference is strength. There are some road stems that will say they are not recommended for downhill/freeride type riding, but are ok for cyclocross. Road stems will also come in a larger range of angles.

Just be sure that when you are looking at stems, you match the clamp of the stem to your steerer tube (1" or 1 1/4" usually) and to the diameter of the bars that you are using.