GRIZZLY YEARS: In Search of the American Wilderness by Doug Peacock (Henry Holt: $12.95; 288 pp.). When the author returned from Vietnam, badly scarred psychologically by the mayhem he had witnessed, he sought refuge in his interest in bears. Peacock interweaves observations in the Rockies with accounts of his service in Southeast Asia. At times, he takes unnecessary chances and risks being mauled by startled bears. The powerful fascination of the mighty grizzly, whose population has been badly depleted by hunting and habitat destruction, enables Peacock to make peace with his demons.