WILLIAM TUCKER'S CAREER has been sliced down the middle. Born in 1935, he made his name in the '60s as one of the younger generation of abstract sculptors associated with Anthony Caro at St. Martin's School of Art in London. In 1974, Tucker published The Language of Sculpture, a history of early modernist sculpture, which rather summed up this phase of his career. It led Albert Elsen to call him a “spokesman for an academic abstract art.” He emigrated to North America in 1976 and wound up on a farm in upstate New York Soon after, he abandoned constructed abstract sculpture, and started to model