A federal judge in Pittsburgh ruled yesterday that most members of a dissident truck drivers' group must halt their violent nine-week-old strike against steel producers within 48 hours.

Leaders of the group, the Fraternal Association of Steel Haulers (FASH) immediately asked for a stay of the order. A ruling on the request is expected today.

FASH's attorney, Paul Boas, of Pittsburgh, said the organization's leadership might not be able to stop the job action -- despite the threat of fines and criminal sentences for individual strikers -- even if yesterday's ruling is upheld.

Thus the immediate effect of Judge Louis Rosenberg's order was to continue the state of confusion and hostility that has marked FASH's dispute with the steel companies and the Teamsters' union since the strike began Nov. 10.

FASH's members, most of whom are Teamster members, parked their big steel-hauling rigs to protest steel companies' refusal to recognize FASH, rather than the Teamsters, as their bargaining agent. The strikers, whose number has been estimated from several hundred to 3,000, have been accused of harassing nonstriking drivers with sniper fire and vandalism in Indiana, Ohio, and Pennsylvania.

In 1971, Rosenberg had enjoined a similar FASH strike. The judge ruled FASH was a businessman's organization, rather than a union, and thus its job action violated antitrust laws.

When the current strike started, seven big steel firms asked Rosenberg to hold FASH in contempt for violating the 1971 injunction. FASH replied that its action was a labor dispute and thus exempt from antitrust prohibitions.

Rosenberg ruled again yesterday that FASH is not a union. He said that leaders of the organization and individual strikers would be liable for fines or imprisonment for contempt of his 1971 order if the strike were not ended by Friday.

However, the judge agreed that FASH does serve as a "labor organization" at a few companiew which have entered into bargaining agreements with FASH rather than the Teamsters. FASH members covered by those agreements can continue to strike with impunity, the judge ruled.

FASH leaders could not say yesterday how many of their members are covered by bargaining contracts, but they said the covered members represent a minority of the strikers.

The steel industry says the FASH strike has delayed shipment of about 10 percent of their tonnage in midwestern states.