I believe this IRS ruling addresses your situation:
"Under the Amended Plan, long-term disability benefits received by an employee who has irrevocably elected, prior to the beginning of the plan year, to have the coverage paid by the Employer on an after-tax basis for the plan year in which the employee becomes disabled are attributable solely to after-tax employee contributions and are excludable from the employee’s gross income under § 104(a)(3).
Under the Amended Plan, long-term disability benefits received by an employee whose coverage is paid by the Employer on a pre-tax basis for the plan year in which the employee becomes disabled are attributable solely to pre-tax Employer contributions and are includible in the employee’s gross income under § 105(a).
These holdings are equally applicable to short-term disability benefits."
https://www.irs.gov/irb/2004-26_IRB#RR-2004-55
"After tax" = the premiums paid by your employer were included in your taxable income
"Pre-tax" = the premiums paid by your employer were not included in your taxable income
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
