From reading about this, I see that one of the symptoms of Ataxia is difficulty swallowing and eating. Is this a problem for you as well? If so, you might talk with your doctor to see how you can increase your fluids and foods in order to deal with the constipation.
Does your doctor think this might be related to any of the meds you are taking for this disorder? If you are comfortable sharing more, how long ago were you diagnosed with Ataxia?