WordNet records no synonym and no antonym for lockstep. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a standard procedure that is followed mindlessly
“the union's support had been in lockstep for years”
a manner of marching in file in which each person's leg moves with and behind the corresponding leg of the person ahead
“the prisoner's ankles were so chained together that they could only march in lockstep”
Synonyms, antonyms and the broader and narrower relations for lockstep come from Princeton University’s WordNet. Full sources and licences.