In his first speech since resigning as secretary of state, Alexander M. Haig Jr. today carefully avoided any direct criticism of American foreign policy but cautioned against extreme positions in American relations with the Soviet Union.

Addressing the American Bar Association's annual convention here, Haig's only discussion of his leave-taking on June 30 took the form of self-deprecating jokes about how a few weeks ago he was "fourth in line" for succession to the presidency but now can't even get "precedence in the supermarket line," and how, after traveling in Air Force jets from continent to continent he now "can't leave home" without his American Express card.

Things are not all that bad with Haig, however. The former secretary of state received the highest speaking fee ever paid by the ABA, $20,000. The fee was a topic of much envious discussion among the 10,000 lawyers gathered here for the convention. Haig drew a full house, 3,600, at his speech.

Haig looked tanned and spoke with his usual dramatic intensity. But he said he was speaking under the "constraint" of current events, particularly those in the Mideast, during which he said President Reagan deserves support from "every American."

Haig said the Middle East situation, though tragic, presents "an unprecedented opportunity" to bring about "total withdrawal of all foreign forces" and a possible expansion of the talks on self-government for Palestinian inhabitants of Israeli-occupied territories at the same time it could result in "a guarantee of security for the northern territories of Israel."

In other convention activities, Supreme Court Justice Lewis F. Powell Jr., like colleague John Paul Stevens earlier, said "fundamental changes" are needed to reduce the caseload problem in the federal courts. He said the dramatic increase in the past decade "may prevent the same degree of judicial care, in selecting cases for full review, than probably existed a decade or more ago."

But in the text of a speech prepared for delivery here tonight, he indicated he did not think the problem went as far as Stevens said it did, though in his text he did not directly mention Stevens' comments. Stevens said Friday that court clerks and "anonymous" administrative staff were increasingly performing tasks that should be reserved for the justices.

Powell said today that the court "has not yet been bureaucratized . . . . Judging remains an individual function. Each of us still makes a personal judgment with respect to each petition" and the justices write all opinions themselves.

Powell was not specific in the changes he would recommend. But he spoke generally of the need for legislation to limit the cases reviewed by federal courts after thorough review by state courts and of suggestions for reducing the types of cases the court is required by law to address.