The two principal groups of dissidents within the International Brotherhood of Teamsters are tentatively planning to merge later this year, ending several years of debilitating rivalry within the numerically weak dissident movement.

Officers of the Washington-based Professional Drivers Council (PROD) and the Detroit-based Teamsters for a Democratic Union (TDU) have endorsed a merger of their two groups subject to rank-and-file approval.

The 4,000 to 5,000 members of PROD will vote by mail next month on whether to approve the merger, according to PROD organizer Robert Fram. TDU may take final action on the merger at a rank-and-file convention in Ypslianti, Mich., in early November.

PROD was formed as apart of a truck safety campaign by consumer advocate Ralph Nader but gradually went its own way, concentrating on legislative and legal opposition to alleged unresponsiveness and improprieties on the part of Teamsters officials TDU grew out of efforts to quell dissent at the Teamsters' 1976 convention, and his concentrated on local organization.

Although always a small minority of the 2-million-member Teamsters union, the two groups of election challenges last year to entrenched local gradually grew closer together, joining in a number Teamster leaders.

They were successful in electing key officials in about 10 locals -- including some of the union's about 10 locals -- including some of the union's largest -- in California, Michigan, Pennsylvania, Missouri and British Columbia, Paff said.

Accoring to Paff, the two groups still must work out some organizational problems. But neither Paff nor Fram indicated any major substantive obstacles to the merger.

Meanwhile, the Teamsters' Central States Pension Fund -- under fire from the government for allegedly imprudent handling of its assets -- has returned to some of its old free-spending habits, including purchasing a $3.5 million jet airplane and renting a yacht for a dinner meeting in Miami, according to the Chicago Sun-Times.

The newspaper also said the fund has undergone another administrative shakeup with the resignation of its executive director, John E. Dwyer, and the firing of its controller, Donald Maxfield.

At the same time, the pension funds' directors have discussed ousting the Equitable Life Assurance Society and other firms hired at the government's insistence to manage the fund's assets and have considered resuming making real estate loans that were halted in an earlier clean-up effort, the Sun-Times reported.