You might want to check out the Adult Listing http://www.ssa.gov/disability/professionals/bluebook/ which lists the illnesses that recieve automatic approval. I briefly looked at the Mental Health Issues, Section 12. I did not see in my brief glance through the listing agoraphobia but many of the listed symtpoms appear to be similiar as to what I guess your wife feels.
SSDI is granted on the functional limitations of a person once a medical impairment is diagnosed.
It's completely possible your wife can get SSDI with the support of her treating physicians.
Best of luck,
Trudi