The Locke-Wallace Marital Adjustment Scale (1959) is probably the most widely researched and utilized scale on marital adjustment. It is brief and should be applicable to your sample. Another comprable scale is the Dyadic Adjustment Scale (Spanier, 1976). Both of these can be found in a search of Psych-Info at a university library.
By the way, I'm working in a Behavioral Medicine service in a primary care setting and I'd like to hear what you're up to. Write me at Psychojer@yahoo.com if you're working in a primary care setting as a psychologist.