I won a judgement a while ago and collected some of the judgement via garnishment of the defendants wages since then the defendant has change jobs and I have lost contact of him. Although I do have his Social Security Number how can I legally find out where he is working so I can resume the garnishment procedures?

Private investigator, basically. Or ask someone who knows him or have him followed (by someone who knows what he/she is doing).

The Social Security number is unlikely to be helpful in determining where he works.