My soon-to-be ex-wife says that I'll have to pay child support out of my meager disability benefits. Is this true?
Yes, some disability benefits can be taken to pay child support. But this only applies to Social Security disability insurance benefits (SSDI). (SSDI is the disability benefit for which you have to pay Social Security taxes for years to be eligible for.) For more information, see Nolo's article on how to get a child support order modified due to disability.)
SSI recipients cannot have their monthly SSI disability benefits seized, or any past due SSI benefits (backpay) seized to pay child support arrears. SSI stands for Supplemental Security Income, a needs-based program for low-income and low-resource disabled people.
The reasoning applied by the federal government is this: Since SSI is essentially a public welfare benefit and does not derive not from a claimant's earnings record, SSI benefits cannot be taken for other purposes, just as food stamps and AFDC funds, likewise, cannot be seized. However, SSDI can be seized because it arises from an individual's earning record, more like regular income. (It actually is an insurance benefit paid for from payroll tax deductions (or self-employment tax deductions) from an individual's income.)