I would go to whichever Dr will more likely guarantee you the signature and report you need for your disability insurance. If you go to that family doctor, who hasn't seen you very often and is not as familiar with your health issues, you might want to create a brief health log outlining your daily life and how Lupus is affecting you, plus ways the Rheumatologist has been treating your various symptoms. He/she may also need to see your lab results, x-rays, etc. done by the Rheumatologist in order to better fill out the forms.
I would also have a talk with my employer about how Lupus challenges your daily life and employment, plus explain the purpose of the special insurance and forms you need filled out. The possible problem with going to a new Dr is that Dr could require a "new patient" exam and lab work at one appt. then discuss it with you at another appt BEFORE being willing to fill out that form for your disability insurance policy.
Whatever you do, DO NOT ever go without active health insurance, even if you have to have a temporary COBRA plan. If you go without active insurance and develop additional health problems that are treated then begin a new health insurance policy, there is a good chance they will NOT cover any pre-existing conditions. This happened to us one time and my son's chronic ear infections, bronchitis, strep throat, ear infections, etc. were not insurable for months.