The IRS considers short-term disability to be a type of sick pay. As such, it's generally only considered to be taxable income if your employer paid the premiums in part or in full. It's also taxable if you paid your own premiums (or a portion of them) with pre-tax dollars.
is there an amount in box 1 of the w-2? is box 13 checked?
you could also try contacting the SSA to see what they show.
