Why are they taxing me for my social security disability payments? Aren't they tax exempt?

According to this IRS link,  there are two ways to determine the amount of income to include:

• You can use your current year's income to figure the taxable part of the total benefits received in the current year; or
• You may make an election to figure the taxable part of a lump-sum payment for an earlier year separately, using your income for the earlier year.

You can select the lump-sum election method if it lowers the taxable portion of your benefits:

• Under this method, you refigure the taxable part of all your benefits (including the lump-sum payment) for the earlier year using that year’s income.
• Then you subtract any taxable benefits for that year that you previously reported.
• The remainder is the taxable part of the lump-sum payment. Add it to the taxable part of your benefits for the current year (figured without the lump-sum payment for the earlier year).
• Worksheets in Publication 915, Social Security and Equivalent Railroad Retirement Benefits can help you calculate the taxable portion using this method.

I don't know who told you that back payments were tax exempt because if combined with other income, some of it could be taxable if combined with other taxable income.  If you need further assistance in reporting this in Turbo Tax, please call Tax Support at 1-800-446-8848 for further assistance.