Henrik Ojamaa put Motherwell ahead in the sixth minute but Steven Whittaker equalized three minutes later and McCulloch scored from about six yards (meters) following a long throw from Lee Wallace.
Rangers denied Celtic the title last week when it beat its local rival 3-2 and now has 60 points with six games remaining, six more than third-place Motherwell.
Celtic has 70 points ahead of Sunday's game against St. Johnstone.
Saturday's three other matches all went with the home teams. Dundee United beat Dunfermline 3-0, Hearts beat Aberdeen 3-0 and St. Mirren beat Kilmarnock.