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President Reagan has been good for business - the ACLU's business. The American Civil Liberties Union has prospered in the two years since Reagan became president. ACLU president Norman Dorsen, a New York University law professor, says the group's membership now stands at more than a quarter of a million - a net gain of 75,000 over two years ago. The growth has stemmed from concerns that the Reagan administration has either weakened or been less-than-aggressive in protecting individual liberties. The ACLU went through hard times a few years back when it decided to support the First Amendment rights of a neo-Nazi group to stage a march through the largely Jewish suburb of Skokie, Ill., outside Chicago.

''We suffered some short-term losses after the Skokie decision,'' Dorsen says , ''but it worked to enhance the ACLU's credibility.'' m