Am I obligated to provide my Tax returns to Guardian for my LTD Disability?

Am I obligated to provide my Tax returns to Guardian for my LTD Disability? I just received a letter from Guardian asking for “complete tax return with all schedules and attachments … Please note, pursuant to provisions in your Group Long-Term Disability plan, copies of an insured’s complete tax returns with all schedules and attachments are required as acceptable proof of earnings for the ongoing adjudication of your claim.” My policy documents do not mention anything about “tax returns”. I have been receiving LTD from Guardian for many years. The payments are offset by the SSDI I also receive. My tax returns are filed jointly with my wife. The tax return lists my occupation as “disabled”. I do receive income from dividends and interest from banks and brokerages. My wife is very uncomfortable with the idea of sending out our tax return (with her information) to Guardian. Any opinion/suggestions?