Environmentalists are speculating that Interior Secretary Bruce Babbitt will try to protect 650,000 acres on Arizona’s Shivwets Plateau north of the Grand Canyon during his last 18 months in office. If Arizona’s congressional delegation can’t agree on legislation to do the job, Babbitt is expected to push Pres. Clinton to designate the area as a national monument. Though Babbitt is being investigated by an independent counsel for his role in a 1995 rejection of a proposed Indian casino, friends and colleagues say he has refused to let the probe squelch his enthusiasm for his job.