Only 20% of the companies recently surveyed by the Society for Human Resource Management offer their employees sabbaticals. That's down dramatically from 35% just two years ago. One of the reasons: the stress that worker absences creates for other employees in the current tight labor market. Among the companies that have changed their stance on sabbaticals: Apple Computer and Compaq Computer. Apple last fall ended its program that gave employees with five years of service six weeks of paid time off. And Compaq is phasing out the sabbatical program of Tandem Computer, which it bought in 1998.