Unless Congress extends the deadline, federal workers have only 62 shopping days to pick their lifetime pension plan.

For the 2 million federal workers hired before 1984, the choice is between their current Civil Service Retirement System, or the new Federal Employees Retirement System. Locally, about 300,000 workers must make a pension decision. So far, fewer than 2 percent have shifted to the FERS plan.

The choice is difficult because the two plans are very different.

CSRS was designed for people who make government a lifelong career. Its benefits are based on salary and length of service with pensions indexed to inflation. But because only 30 percent of the people who go to work for Uncle Sam actually retire from government, FERS may be a better deal for many younger employes.

FERS is like a private pension plan with benefits coming from Social Security, a modified civil service annuity, plus earnings from an optional tax-deferred thrift savings plan. Workers under FERS can contribute up to 10 percent of salary to the thrift plan and get a 5 percent match from the government. Employes under CSRS can invest 5 percent of pay into the thrift plan, but get no matching government contribution.

Picking the best pension plan is difficult.

Free help is available at federal offices. Workers can get a free computer analysis of their benefits under both plans. Some federal and postal unions are giving members either free or reduced-rate computer analyses. Also, the government has provided employes with do-it-yourself workbooks to help figure their benefits.

Many local private firms are offering personalized computer benefits analyses for a fee. Participants fill out a questionnaire and pay a fee and are sent a benefits sheet.

Firms that are providing the computer analysis are:PIPS, fee $32. Telephone 532-1631.

American Financial Service, $12 (fee may go up soon). Telephone, 451-1300.

SORT, $32. Telephone 354-837.

Chambers Associates, $25. Telephone, 857-0670.

The October issue of Washington Consumers Checkbook magazine also rated the computer firms, as well as the government's free-advice operation. For copies of Checkbook call 347-9612.Health Insurance Nightmare

On Nov. 9 federal workers and retirees begin a monthlong search for a health plan that will cover them and their families next year without costing them an arm and a leg. While some of the 400 plans in the federal health program are holding the line or cutting premiums next year, the average premium increase in 1988 will be a record 31 percent.

To help federal workers and retirees decide which plan is best for them, we will have a series of articles during the open season, listing best buys as rated by insurance experts. On Sunday we'll have a sneak preview of health plan ratings using a new system designed by Government Retirement Benefits Inc. of Alexandria. GRB rates plans based on their catastrophic coverage protection, customer satisfaction and the total price to the subscriber.

At 1 p.m. tomorrow on WNTR radio (1050 AM), Federal Times reporter Susan Kellam will talk about the health insurance situation with federal and private sector experts. The key question: Why a 31 percent increase?