The divorce rate in Illinois rose from about 1958 to 1979, then began to fall. By 2009 the divorce was approaching the same rate as in the early 1960s.
The most dramatic change over the past 50 years is the decline in the rate of marriage. The highest rate of marriage was in 1973. The rate has steadily declined since then. The baby boom children were entering their late teens between 1965-1975. Clearly they were getting married at a high rate.
These numbers were based on reports produced by the Illinois Department of Public Health. The original report can be found here: http://www.idph.state.il.us/health/bdmd/marr_div_annul.htm